Given a string s, determine if it contains any palindrome of length exactly 100 as a subsequence. If it has any, print any one of them. If it doesn't have any, print a palindrome that is a subsequence of s and is as long as possible.

Input

The only line of the input contains one string s of length n (1 ≤ n ≤ 5·104) containing only lowercase English letters.

Output

If s contains a palindrome of length exactly 100 as a subsequence, print any palindrome of length 100 which is a subsequence of s. If s doesn't contain any palindromes of length exactly 100, print a palindrome that is a subsequence of s and is as long as possible.

If there exists multiple answers, you are allowed to print any of them.

Examples

Input

bbbabcbbb


Output

bbbcbbb


Input

rquwmzexectvnbanemsmdufrg


Output

rumenanemur

Note

A subsequence of a string is a string that can be derived from it by deleting some characters without changing the order of the remaining characters. A palindrome is a string that reads the same forward or backward.
import java.io.*;
import java.util.*;
import java.util.stream.Stream;

public class Main implements Runnable {
    static final int MOD = (int) 1e9 + 7;
    static final int MI = (int) 1e9;
    static final long ML = (long) 1e18;
    static final Reader in = new Reader();
    static final PrintWriter out = new PrintWriter(System.out);
    StringBuilder answer = new StringBuilder();

    public static void main(String[] args) {
        new Thread(null, new Main(), "persefone", 1 << 32).start();
    }

    @Override
    public void run() {
        solve();
        printf();
        flush();
    }


    void solve() {
        char[] s = " ".concat(in.next()).toCharArray();
        int n = s.length - 1;
        if (n >= 2600) {
            int[] cnt = new int[26];
            for (int i = 1; i <= n; i++) {
                cnt[s[i] - 'a']++;
            }
            for (int i = 0; i < 26; i++) {
                if (cnt[i] >= 100) {
                    for (int j = 0; j < 100; j++) {
                        add((char) (i + 97));
                    }
                    return;
                }
            }
        }
        
        int[][] dp = new int[n + 1][n + 1];
        boolean[][] can = new boolean[n + 1][n + 1];
        for (int j = 1; j <= n; j++) {
            dp[j][j] = 1;
            for (int i = j - 1; i > 0; i--) {
                dp[i][j] = Math.max(dp[i + 1][j], dp[i][j - 1]);
                if (s[i] == s[j]) {
                    can[i][j] = true;
                    dp[i][j] = Math.max(dp[i][j], 2 + dp[i + 1][j - 1]);
                }
            }
        }
        if (dp[1][n] == 1) {
            printf(s[1]);
            return;
        }
        int max = dp[1][n];
        for (int i = 1; i <= n; i++) {
            for (int j = i + 1; j <= n; j++) {
                if (!can[i][j]) {
                    dp[i][j] = 0;
                }
            }
        }
        getAns(dp, s, 1, n, max);
        char[] res = new char[max];
        for (int i = 0; i < ans.length(); i++) {
            if (i != max - 1) {
                res[i] = res[max - i - 1] = ans.charAt(i);
            } else {
                if (ans.length() * 2 == max) {
                    res[i] = res[i + 1] = ans.charAt(i);
                } else {
                    res[i] = ans.charAt(i);
                }
            }
        }
        if (max <= 99) {
            printf(res);
            return;
        }

        char[] fres = new char[100];
        for (int i = 0; i < 50; i++) {
            fres[i] = fres[99 - i] = res[i];
        }
        printf(fres);

    }

    String ans = "";
    void getAns(int[][] dp, char[] s, int low, int high, int len) {
        if (low > high) return ;
        outer:
        for (int j = high; j > 0; j--) {
            for (int i = low; i <= j; i++) {
                if (dp[i][j] == len) {
                    ans += s[j];
                    getAns(dp, s, i + 1, j - 1, len - 2);
                    break outer;
                }
            }
        }
    }

    void printf() {
        out.print(answer);
    }

    void close() {
        out.close();
    }

    void flush() {
        out.flush();
    }

    void printf(Stream<?> str) {
        str.forEach(o -> add(o, " "));
        add("\n");
    }


    void printf(Object... obj) {
        printf(false, obj);
    }

    void printfWithDescription(Object... obj) {
        printf(true, obj);
    }


    private void printf(boolean b, Object... obj) {

        if (obj.length > 1) {
            for (int i = 0; i < obj.length; i++) {
                if (b) add(obj[i].getClass().getSimpleName(), " - ");
                if (obj[i] instanceof Collection<?>) {
                    printf((Collection<?>) obj[i]);
                } else if (obj[i] instanceof int[][]) {
                    printf((int[][]) obj[i]);
                } else if (obj[i] instanceof long[][]) {
                    printf((long[][]) obj[i]);
                } else if (obj[i] instanceof double[][]) {
                    printf((double[][]) obj[i]);
                } else printf(obj[i]);
            }
            return;
        }

        if (b) add(obj[0].getClass().getSimpleName(), " - ");
        printf(obj[0]);
    }

    void printf(Object o) {
        if (o instanceof int[])
            printf(Arrays.stream((int[]) o).boxed());
        else if (o instanceof char[])
            printf(new String((char[]) o));
        else if (o instanceof long[])
            printf(Arrays.stream((long[]) o).boxed());
        else if (o instanceof double[])
            printf(Arrays.stream((double[]) o).boxed());
        else if (o instanceof boolean[]) {
            for (boolean b : (boolean[]) o) add(b, " ");
            add("\n");
        } else
            add(o, "\n");
    }

    void printf(int[]... obj) {
        for (int i = 0; i < obj.length; i++) printf(obj[i]);
    }

    void printf(long[]... obj) {
        for (int i = 0; i < obj.length; i++) printf(obj[i]);
    }

    void printf(double[]... obj) {
        for (int i = 0; i < obj.length; i++) printf(obj[i]);
    }

    void printf(boolean[]... obj) {
        for (int i = 0; i < obj.length; i++) printf(obj[i]);
    }

    void printf(Collection<?> col) {
        printf(col.stream());
    }

    <T, K> void add(T t, K k) {
        if (t instanceof Collection<?>) {
            ((Collection<?>) t).forEach(i -> add(i, " "));
        } else if (t instanceof Object[]) {
            Arrays.stream((Object[]) t).forEach(i -> add(i, " "));
        } else
            add(t);
        add(k);
    }


    <T> void add(T t) {
        answer.append(t);
    }

    static class Reader {
        private BufferedReader br;
        private StringTokenizer st;

        Reader() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        Reader(String fileName) throws FileNotFoundException {
            br = new BufferedReader(new FileReader(fileName));
        }

        boolean isReady() throws IOException {
            return br.ready();
        }

        String next() {
            try {
                while (st == null || !st.hasMoreTokens()) {
                    st = new StringTokenizer(br.readLine());
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
            return st.nextToken();
        }

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

        int[] nextIntArray(int n) {
            int[] arr = new int[n];
            for (int i = 0; i < n; i++)
                arr[i] = nextInt();
            return arr;
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        String nextLine() {
            String s = "";
            try {
                s = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return s;
        }
    }
}