You are given a string s, consisting of lowercase English letters, and the integer m.

One should choose some symbols from the given string so that any contiguous subsegment of length m has at least one selected symbol. Note that here we choose positions of symbols, not the symbols themselves.

Then one uses the chosen symbols to form a new string. All symbols from the chosen position should be used, but we are allowed to rearrange them in any order.

Formally, we choose a subsequence of indices 1 ≤ i1 < i2 < ... < it ≤ |s|. The selected sequence must meet the following condition: for every j such that 1 ≤ j ≤ |s| - m + 1, there must be at least one selected index that belongs to the segment [j, j + m - 1], i.e. there should exist a k from 1 to t, such that j ≤ ik ≤ j + m - 1.

Then we take any permutation p of the selected indices and form a new string sip1sip2... sipt.

Find the lexicographically smallest string, that can be obtained using this procedure.

Input

The first line of the input contains a single integer m (1 ≤ m ≤ 100 000).

The second line contains the string s consisting of lowercase English letters. It is guaranteed that this string is non-empty and its length doesn't exceed 100 000. It is also guaranteed that the number m doesn't exceed the length of the string s.

Output

Print the single line containing the lexicographically smallest string, that can be obtained using the procedure described above.

Examples

Input

3
cbabc


Output

a


Input

2
abcab


Output

aab


Input

3
bcabcbaccba


Output

aaabb

Note

In the first sample, one can choose the subsequence {3} and form a string "a".

In the second sample, one can choose the subsequence {1, 2, 4} (symbols on this positions are 'a', 'b' and 'a') and rearrange the chosen symbols to form a string "aab".
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Arrays;
import java.io.IOException;
import java.io.Serializable;
import java.io.UncheckedIOException;
import java.io.Closeable;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) throws Exception {
        Thread thread = new Thread(null, new TaskAdapter(), "", 1 << 29);
        thread.start();
        thread.join();
    }

    static class TaskAdapter implements Runnable {
        @Override
        public void run() {
            InputStream inputStream = System.in;
            OutputStream outputStream = System.out;
            FastInput in = new FastInput(inputStream);
            FastOutput out = new FastOutput(outputStream);
            DDenseSubsequence solver = new DDenseSubsequence();
            solver.solve(1, in, out);
            out.close();
        }
    }

    static class DDenseSubsequence {
        int[] dp;
        int inf = (int) 1e9;
        IntegerMinQueue queue;

        public int solve(int[] s, int m) {
            int n = s.length;
            queue.clear();
            for (int i = 0; i < m; i++) {
                queue.addLast(0);
            }
            for (int i = 0; i < n; i++) {
                if (s[i] > 0) {
                    dp[i] = queue.min() + 1;
                } else {
                    dp[i] = inf;
                }
                if (s[i] == 2) {
                    queue.clear();
                }
                queue.addLast(dp[i]);
                if (queue.size() > m) {
                    queue.removeFirst();
                }
            }
            return queue.min();
        }

        public void solve(int testNumber, FastInput in, FastOutput out) {
            int m = in.ri();
            char[] s = new char[(int) 1e5];
            int n = in.rs(s);
            s = Arrays.copyOf(s, n);
            int[] cast = new int[n];
            dp = new int[n];
            queue = new IntegerMinQueue(n, IntegerComparator.NATURE_ORDER);
            for (int i = 'a'; i <= 'z'; i++) {
                for (int j = 0; j < n; j++) {
                    cast[j] = s[j] == i ? 1 : s[j] < i ? 2 : 0;
                }
                int res = solve(cast, m);
                if (res >= inf) {
                    continue;
                }
                IntegerArrayList seq = new IntegerArrayList(res);
                for (int j = 0; j < n; j++) {
                    if (s[j] < i) {
                        seq.add(s[j]);
                    }
                }
                while (seq.size() < res) {
                    seq.add(i);
                }
                seq.sort();
                for (int x : seq.toArray()) {
                    out.append((char) x);
                }
                return;
            }
        }

    }

    static class SequenceUtils {
        public static boolean equal(int[] a, int al, int ar, int[] b, int bl, int br) {
            if ((ar - al) != (br - bl)) {
                return false;
            }
            for (int i = al, j = bl; i <= ar; i++, j++) {
                if (a[i] != b[j]) {
                    return false;
                }
            }
            return true;
        }

    }

    static class Randomized {
        public static void shuffle(int[] data, int from, int to) {
            to--;
            for (int i = from; i <= to; i++) {
                int s = nextInt(i, to);
                int tmp = data[i];
                data[i] = data[s];
                data[s] = tmp;
            }
        }

        public static int nextInt(int l, int r) {
            return RandomWrapper.INSTANCE.nextInt(l, r);
        }

    }

    static class FastInput {
        private final InputStream is;
        private byte[] buf = new byte[1 << 13];
        private int bufLen;
        private int bufOffset;
        private int next;

        public FastInput(InputStream is) {
            this.is = is;
        }

        private int read() {
            while (bufLen == bufOffset) {
                bufOffset = 0;
                try {
                    bufLen = is.read(buf);
                } catch (IOException e) {
                    bufLen = -1;
                }
                if (bufLen == -1) {
                    return -1;
                }
            }
            return buf[bufOffset++];
        }

        public void skipBlank() {
            while (next >= 0 && next <= 32) {
                next = read();
            }
        }

        public int ri() {
            return readInt();
        }

        public int readInt() {
            int sign = 1;

            skipBlank();
            if (next == '+' || next == '-') {
                sign = next == '+' ? 1 : -1;
                next = read();
            }

            int val = 0;
            if (sign == 1) {
                while (next >= '0' && next <= '9') {
                    val = val * 10 + next - '0';
                    next = read();
                }
            } else {
                while (next >= '0' && next <= '9') {
                    val = val * 10 - next + '0';
                    next = read();
                }
            }

            return val;
        }

        public int rs(char[] data, int offset) {
            return readString(data, offset);
        }

        public int rs(char[] data) {
            return rs(data, 0);
        }

        public int readString(char[] data, int offset) {
            skipBlank();

            int originalOffset = offset;
            while (next > 32) {
                data[offset++] = (char) next;
                next = read();
            }

            return offset - originalOffset;
        }

    }

    static interface IntegerStack {
    }

    static class IntegerDequeImpl implements IntegerDeque {
        private int[] data;
        private int bpos;
        private int epos;
        private static final int[] EMPTY = new int[0];
        private int n;

        public IntegerDequeImpl(int cap) {
            if (cap == 0) {
                data = EMPTY;
            } else {
                data = new int[cap];
            }
            bpos = 0;
            epos = 0;
            n = cap;
        }

        private void expandSpace(int len) {
            while (n < len) {
                n = Math.max(n + 10, n * 2);
            }
            int[] newData = new int[n];
            if (bpos <= epos) {
                if (bpos < epos) {
                    System.arraycopy(data, bpos, newData, 0, epos - bpos);
                }
            } else {
                System.arraycopy(data, bpos, newData, 0, data.length - bpos);
                System.arraycopy(data, 0, newData, data.length - bpos, epos);
            }
            epos = size();
            bpos = 0;
            data = newData;
        }

        public IntegerIterator iterator() {
            return new IntegerIterator() {
                int index = bpos;


                public boolean hasNext() {
                    return index != epos;
                }


                public int next() {
                    int ans = data[index];
                    index = IntegerDequeImpl.this.next(index);
                    return ans;
                }
            };
        }

        public int removeFirst() {
            int ans = data[bpos];
            bpos = next(bpos);
            return ans;
        }

        public int removeLast() {
            int ans = data[last(epos)];
            epos = last(epos);
            return ans;
        }

        public void addLast(int x) {
            ensureMore();
            data[epos] = x;
            epos = next(epos);
        }

        public int peekFirst() {
            return data[bpos];
        }

        public int peekLast() {
            return data[last(epos)];
        }

        public void clear() {
            bpos = epos = 0;
        }

        private int last(int x) {
            return (x == 0 ? n : x) - 1;
        }

        private int next(int x) {
            return x + 1 >= n ? 0 : x + 1;
        }

        private void ensureMore() {
            if (next(epos) == bpos) {
                expandSpace(n + 1);
            }
        }

        public int size() {
            int ans = epos - bpos;
            if (ans < 0) {
                ans += data.length;
            }
            return ans;
        }

        public boolean isEmpty() {
            return bpos == epos;
        }

        public String toString() {
            StringBuilder builder = new StringBuilder();
            for (IntegerIterator iterator = iterator(); iterator.hasNext(); ) {
                builder.append(iterator.next()).append(' ');
            }
            return builder.toString();
        }

    }

    static interface IntegerComparator {
        public static final IntegerComparator NATURE_ORDER = (a, b) -> Integer.compare(a, b);

        public int compare(int a, int b);

    }

    static strictfp class MersenneTwisterFast implements Serializable, Cloneable {
        private static final int N = 624;
        private static final int M = 397;
        private static final int MATRIX_A = 0x9908b0df;
        private static final int UPPER_MASK = 0x80000000;
        private static final int LOWER_MASK = 0x7fffffff;
        private static final int TEMPERING_MASK_B = 0x9d2c5680;
        private static final int TEMPERING_MASK_C = 0xefc60000;
        private int[] mt;
        private int mti;
        private int[] mag01;
        private boolean __haveNextNextGaussian;

        public Object clone() {
            try {
                MersenneTwisterFast f = (MersenneTwisterFast) (super.clone());
                f.mt = (int[]) (mt.clone());
                f.mag01 = (int[]) (mag01.clone());
                return f;
            } catch (CloneNotSupportedException e) {
                throw new InternalError();
            } // should never happen
        }

        public MersenneTwisterFast() {
            this(System.currentTimeMillis());
        }

        public MersenneTwisterFast(long seed) {
            setSeed(seed);
        }

        public MersenneTwisterFast(int[] array) {
            setSeed(array);
        }

        public void setSeed(long seed) {
            // Due to a bug in java.util.Random clear up to 1.2, we're
            // doing our own Gaussian variable.
            __haveNextNextGaussian = false;

            mt = new int[N];

            mag01 = new int[2];
            mag01[0] = 0x0;
            mag01[1] = MATRIX_A;

            mt[0] = (int) (seed & 0xffffffff);
            for (mti = 1; mti < N; mti++) {
                mt[mti] =
                        (1812433253 * (mt[mti - 1] ^ (mt[mti - 1] >>> 30)) + mti);
                /* See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier. */
                /* In the previous versions, MSBs of the seed affect   */
                /* only MSBs of the array mt[].                        */
                /* 2002/01/09 modified by Makoto Matsumoto             */
                // mt[mti] &= 0xffffffff;
                /* for >32 bit machines */
            }
        }

        public void setSeed(int[] array) {
            if (array.length == 0)
                throw new IllegalArgumentException("Array length must be greater than zero");
            int i, j, k;
            setSeed(19650218);
            i = 1;
            j = 0;
            k = (N > array.length ? N : array.length);
            for (; k != 0; k--) {
                mt[i] = (mt[i] ^ ((mt[i - 1] ^ (mt[i - 1] >>> 30)) * 1664525)) + array[j] + j; /* non linear */
                // mt[i] &= 0xffffffff; /* for WORDSIZE > 32 machines */
                i++;
                j++;
                if (i >= N) {
                    mt[0] = mt[N - 1];
                    i = 1;
                }
                if (j >= array.length) j = 0;
            }
            for (k = N - 1; k != 0; k--) {
                mt[i] = (mt[i] ^ ((mt[i - 1] ^ (mt[i - 1] >>> 30)) * 1566083941)) - i; /* non linear */
                // mt[i] &= 0xffffffff; /* for WORDSIZE > 32 machines */
                i++;
                if (i >= N) {
                    mt[0] = mt[N - 1];
                    i = 1;
                }
            }
            mt[0] = 0x80000000; /* MSB is 1; assuring non-zero initial array */
        }

        public int nextInt(int n) {
            if (n <= 0)
                throw new IllegalArgumentException("n must be positive, got: " + n);

            if ((n & -n) == n)  // i.e., n is a power of 2
            {
                int y;

                if (mti >= N)   // generate N words at one time
                {
                    int kk;
                    final int[] mt = this.mt; // locals are slightly faster
                    final int[] mag01 = this.mag01; // locals are slightly faster

                    for (kk = 0; kk < N - M; kk++) {
                        y = (mt[kk] & UPPER_MASK) | (mt[kk + 1] & LOWER_MASK);
                        mt[kk] = mt[kk + M] ^ (y >>> 1) ^ mag01[y & 0x1];
                    }
                    for (; kk < N - 1; kk++) {
                        y = (mt[kk] & UPPER_MASK) | (mt[kk + 1] & LOWER_MASK);
                        mt[kk] = mt[kk + (M - N)] ^ (y >>> 1) ^ mag01[y & 0x1];
                    }
                    y = (mt[N - 1] & UPPER_MASK) | (mt[0] & LOWER_MASK);
                    mt[N - 1] = mt[M - 1] ^ (y >>> 1) ^ mag01[y & 0x1];

                    mti = 0;
                }

                y = mt[mti++];
                y ^= y >>> 11;                          // TEMPERING_SHIFT_U(y)
                y ^= (y << 7) & TEMPERING_MASK_B;       // TEMPERING_SHIFT_S(y)
                y ^= (y << 15) & TEMPERING_MASK_C;      // TEMPERING_SHIFT_T(y)
                y ^= (y >>> 18);                        // TEMPERING_SHIFT_L(y)

                return (int) ((n * (long) (y >>> 1)) >> 31);
            }

            int bits, val;
            do {
                int y;

                if (mti >= N)   // generate N words at one time
                {
                    int kk;
                    final int[] mt = this.mt; // locals are slightly faster
                    final int[] mag01 = this.mag01; // locals are slightly faster

                    for (kk = 0; kk < N - M; kk++) {
                        y = (mt[kk] & UPPER_MASK) | (mt[kk + 1] & LOWER_MASK);
                        mt[kk] = mt[kk + M] ^ (y >>> 1) ^ mag01[y & 0x1];
                    }
                    for (; kk < N - 1; kk++) {
                        y = (mt[kk] & UPPER_MASK) | (mt[kk + 1] & LOWER_MASK);
                        mt[kk] = mt[kk + (M - N)] ^ (y >>> 1) ^ mag01[y & 0x1];
                    }
                    y = (mt[N - 1] & UPPER_MASK) | (mt[0] & LOWER_MASK);
                    mt[N - 1] = mt[M - 1] ^ (y >>> 1) ^ mag01[y & 0x1];

                    mti = 0;
                }

                y = mt[mti++];
                y ^= y >>> 11;                          // TEMPERING_SHIFT_U(y)
                y ^= (y << 7) & TEMPERING_MASK_B;       // TEMPERING_SHIFT_S(y)
                y ^= (y << 15) & TEMPERING_MASK_C;      // TEMPERING_SHIFT_T(y)
                y ^= (y >>> 18);                        // TEMPERING_SHIFT_L(y)

                bits = (y >>> 1);
                val = bits % n;
            } while (bits - val + (n - 1) < 0);
            return val;
        }

    }

    static class FastOutput implements AutoCloseable, Closeable, Appendable {
        private static final int THRESHOLD = 1 << 13;
        private final Writer os;
        private StringBuilder cache = new StringBuilder(THRESHOLD * 2);

        public FastOutput append(CharSequence csq) {
            cache.append(csq);
            return this;
        }

        public FastOutput append(CharSequence csq, int start, int end) {
            cache.append(csq, start, end);
            return this;
        }

        private void afterWrite() {
            if (cache.length() < THRESHOLD) {
                return;
            }
            flush();
        }

        public FastOutput(Writer os) {
            this.os = os;
        }

        public FastOutput(OutputStream os) {
            this(new OutputStreamWriter(os));
        }

        public FastOutput append(char c) {
            cache.append(c);
            afterWrite();
            return this;
        }

        public FastOutput flush() {
            try {
                os.append(cache);
                os.flush();
                cache.setLength(0);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
            return this;
        }

        public void close() {
            flush();
            try {
                os.close();
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        }

        public String toString() {
            return cache.toString();
        }

    }

    static class IntegerMinQueue {
        IntegerDequeImpl minQueue;
        IntegerDequeImpl data;
        IntegerComparator comparator;

        public IntegerMinQueue(int cap, IntegerComparator comparator) {
            minQueue = new IntegerDequeImpl(cap);
            data = new IntegerDequeImpl(cap);
            this.comparator = comparator;
        }

        public void clear() {
            minQueue.clear();
            data.clear();
        }

        public void addLast(int val) {
            data.addLast(val);
            while (!minQueue.isEmpty() && comparator.compare(minQueue.peekLast(), val) > 0) {
                minQueue.removeLast();
            }
            minQueue.addLast(val);
        }

        public int removeFirst() {
            assert size() > 0;
            int val = data.removeFirst();
            if (minQueue.peekFirst() == val) {
                minQueue.removeFirst();
            }
            return val;
        }

        public int size() {
            return data.size();
        }

        public int min() {
            assert size() > 0;
            return minQueue.peekFirst();
        }

    }

    static interface IntegerDeque extends IntegerStack {
    }

    static class RandomWrapper {
        private MersenneTwisterFast random;
        public static final RandomWrapper INSTANCE = new RandomWrapper();

        public RandomWrapper() {
            this(new MersenneTwisterFast());
        }

        public RandomWrapper(MersenneTwisterFast random) {
            this.random = random;
        }

        public RandomWrapper(long seed) {
            this(new MersenneTwisterFast(seed));
        }

        public int nextInt(int l, int r) {
            return random.nextInt(r - l + 1) + l;
        }

    }

    static interface IntegerIterator {
        boolean hasNext();

        int next();

    }

    static class IntegerArrayList implements Cloneable {
        private int size;
        private int cap;
        private int[] data;
        private static final int[] EMPTY = new int[0];

        public IntegerArrayList(int cap) {
            this.cap = cap;
            if (cap == 0) {
                data = EMPTY;
            } else {
                data = new int[cap];
            }
        }

        public IntegerArrayList(int[] data) {
            this(0);
            addAll(data);
        }

        public IntegerArrayList(IntegerArrayList list) {
            this.size = list.size;
            this.cap = list.cap;
            this.data = Arrays.copyOf(list.data, size);
        }

        public IntegerArrayList() {
            this(0);
        }

        public void ensureSpace(int req) {
            if (req > cap) {
                while (cap < req) {
                    cap = Math.max(cap + 10, 2 * cap);
                }
                data = Arrays.copyOf(data, cap);
            }
        }

        public void add(int x) {
            ensureSpace(size + 1);
            data[size++] = x;
        }

        public void addAll(int[] x) {
            addAll(x, 0, x.length);
        }

        public void addAll(int[] x, int offset, int len) {
            ensureSpace(size + len);
            System.arraycopy(x, offset, data, size, len);
            size += len;
        }

        public void addAll(IntegerArrayList list) {
            addAll(list.data, 0, list.size);
        }

        public void sort() {
            if (size <= 1) {
                return;
            }
            Randomized.shuffle(data, 0, size);
            Arrays.sort(data, 0, size);
        }

        public int size() {
            return size;
        }

        public int[] toArray() {
            return Arrays.copyOf(data, size);
        }

        public String toString() {
            return Arrays.toString(toArray());
        }

        public boolean equals(Object obj) {
            if (!(obj instanceof IntegerArrayList)) {
                return false;
            }
            IntegerArrayList other = (IntegerArrayList) obj;
            return SequenceUtils.equal(data, 0, size - 1, other.data, 0, other.size - 1);
        }

        public int hashCode() {
            int h = 1;
            for (int i = 0; i < size; i++) {
                h = h * 31 + Integer.hashCode(data[i]);
            }
            return h;
        }

        public IntegerArrayList clone() {
            IntegerArrayList ans = new IntegerArrayList();
            ans.addAll(this);
            return ans;
        }

    }
}

