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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.Reader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.StringTokenizer;

public class D {

    static boolean validate(ArrayList<Integer> taken, int m, int n) {
        if (taken.size() == 0)
            return false;
        for (int i = 0; i < taken.size() - 1; i++) {
            if (taken.get(i + 1) - taken.get(i) > m)
                return false;
        }
        if (taken.get(0) >= m)
            return false;
        if (n - taken.get(taken.size() - 1) > m)
            return false;
        return true;
    }

    static ArrayList<Integer> check(char[] s, char last, int m) {
        int lastGood = -1;
        int prevCurrent = -1;
        ArrayList<Integer> taken = new ArrayList<>();
        for (int i = 0; i < s.length; i++) {
            if (i - lastGood > m) {
                if (prevCurrent != -1) {
                    lastGood = prevCurrent;
                    taken.add(prevCurrent);
                }
            }
            if (s[i] < last) {
                taken.add(i);
                lastGood = i;
            }
            if (s[i] == last) {
                prevCurrent = i;
            }
        }
        if (validate(taken, m, s.length)) {
            return taken;
        }
        if (prevCurrent != -1) {
            taken.add(prevCurrent);
            if (validate(taken, m, s.length)) {
                return taken;
            }
        }
        return null;
    }


    public static void main(String[] args) throws IOException {
        MyScanner in = new MyScanner(new InputStreamReader(System.in));
        int m = in.nextInt();
        char[] s = in.next().toCharArray();
        for (char ch = 'a'; ch <= 'z'; ch++) {
            ArrayList<Integer> positions = check(s, ch, m);
            if (positions != null) {
                char[] ans = new char[positions.size()];
                for (int i = 0; i < positions.size(); i++) {
                    ans[i] = s[positions.get(i)];
                }
                Arrays.sort(ans);
                System.out.println(new String(ans));
                System.exit(0);
            }
        }
    }

    public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;

        public MyScanner(Reader r) {
            br = new BufferedReader(r);
        }

        public String next() throws IOException {
            while (st == null || !st.hasMoreTokens()) {
                st = new StringTokenizer(br.readLine());
            }
            return st.nextToken();
        }

        public int nextInt() throws IOException {
            return Integer.parseInt(next());
        }
    }
}
