This problem's actual name, "Lexicographically Largest Palindromic Subsequence" is too long to fit into the page headline.

You are given string s consisting of lowercase English letters only. Find its lexicographically largest palindromic subsequence.

We'll call a non-empty string s[p1p2... pk] = sp1sp2... spk (1 ≤  p1 < p2 < ... < pk ≤  |s|) a subsequence of string s = s1s2... s|s|, where |s| is the length of string s. For example, strings "abcb", "b" and "abacaba" are subsequences of string "abacaba".

String x = x1x2... x|x| is lexicographically larger than string y = y1y2... y|y| if either |x| > |y| and x1 = y1, x2 = y2, ..., x|y| = y|y|, or there exists such number r (r < |x|, r < |y|) that x1 = y1, x2 = y2, ..., xr = yr and xr + 1 > yr + 1. Characters in the strings are compared according to their ASCII codes. For example, string "ranger" is lexicographically larger than string "racecar" and string "poster" is lexicographically larger than string "post".

String s = s1s2... s|s| is a palindrome if it matches string rev(s) = s|s|s|s| - 1... s1. In other words, a string is a palindrome if it reads the same way from left to right and from right to left. For example, palindromic strings are "racecar", "refer" and "z".

Input

The only input line contains a non-empty string s consisting of lowercase English letters only. Its length does not exceed 10.

Output

Print the lexicographically largest palindromic subsequence of string s.

Examples

Input

radar


Output

rr


Input

bowwowwow


Output

wwwww


Input

codeforces


Output

s


Input

mississipp


Output

ssss

Note

Among all distinct subsequences of string "radar" the following ones are palindromes: "a", "d", "r", "aa", "rr", "ada", "rar", "rdr", "raar" and "radar". The lexicographically largest of them is "rr".
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.Writer;
import java.util.InputMismatchException;

public class Main {
    public static void main(String[] args) throws IOException {
        InputStream stdin = System.in;
        OutputStream stdout = System.out;

        /*
         * InputStream fileIn = new FileInputStream(new File("input.txt"));
         * OutputStream fileOut = new FileOutputStream(new File("output.txt"));
         */

        InputReader in = new InputReader(stdin);
        OutputWriter out = new OutputWriter(stdout);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    }
}

class Task {
    public void solve(int testNumber, InputReader in, OutputWriter out) {
        int max = 0;
        String s = in.readString();
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) - 'a' > max) {
                max = s.charAt(i) - 'a';
            }
        }
        int count = 0;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == (char)('a' + max)) {
                count++;
            }
        }
        for (int i = 0; i < count; i++) {
            System.out.print((char)('a' + max));
        }
        System.out.println();
    
    }
}

class InputReader {

    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;

    public InputReader(InputStream stream) {
        this.stream = stream;
    }

    public int read() {
        if (numChars == -1)
            throw new InputMismatchException();
        if (curChar >= numChars) {
            curChar = 0;
            try {
                numChars = stream.read(buf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (numChars <= 0)
                return -1;
        }
        return buf[curChar++];
    }

    public int readInt() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        int res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c - '0';
            c = read();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public long readLong() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        long res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c - '0';
            c = read();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public String readString() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        StringBuffer res = new StringBuffer();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }
    
    public char readCharacter() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        return (char) c;
    }

    public static boolean isSpaceChar(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

}

class OutputWriter {
    private final PrintWriter writer;

    public OutputWriter(OutputStream outputStream) {
        writer = new PrintWriter(outputStream);
    }

    public OutputWriter(Writer writer) {
        this.writer = new PrintWriter(writer);
    }

    public void print(Object... objects) {
        for (int i = 0; i < objects.length; i++) {
            if (i != 0)
                writer.print(' ');
            writer.print(objects[i]);
        }
    }

    public void printLine(Object... objects) {
        print(objects);
        writer.println();
    }

    public void close() {
        writer.close();
    }
}

class StringUtil {
    public static int[] prefixFunction(String s) {
        int n = s.length();
        int[] p = new int[n];
        for (int i = 1; i < n; i++) {
            int k = p[i - 1];
            while (k > 0 && s.charAt(k) != s.charAt(i)) {
                k = p[k - 1];
            }
            p[i] = k + (s.charAt(k) == s.charAt(i) ? 1 : 0);
        }
        return p;
    }

    public static int kmpMatcher(String s, String pattern) {
        int m = pattern.length();
        if (m == 0) {
            return 0;
        }
        int n = s.length();
        int[] p = prefixFunction(pattern);
        int k = 0;
        for (int i = 0; i < n; i++) {
            while (k > 0 && s.charAt(i) != pattern.charAt(k)) {
                k = p[k - 1];
            }
            if (s.charAt(i) == pattern.charAt(k)) {
                k++;
            }
            if (k == m) {
                return i - m + 1;
            }
        }
        return -1;
    }
}