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.BufferedWriter;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.ObjectInputStream.GetField;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.InputMismatchException;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;
import java.util.TreeMap;
import java.util.TreeSet;

public class Q2 {
	static long MOD = 1000000007;
	static boolean b[];
	static ArrayList<Integer>[] amp;
	static int sum[],dist[];
	static long ans = 0;
	static int p = 0;
	static FasterScanner sc = new FasterScanner();
	static Queue<Integer> q = new LinkedList<>();
	static ArrayList<String>[] arr;
	static ArrayList<Integer> parent = new ArrayList<>();
	static BufferedWriter log;
	static HashMap<Long,Long> hm;
	static HashSet<String> hs = new HashSet<>();
	static Stack<Integer> s = new Stack<>();
	static Pair prr[];
	public static void main(String[] args) throws IOException {
		log = new BufferedWriter(new OutputStreamWriter(System.out));
		String str = sc.nextLine();
		int arr[] = new int[26];
		for(int i = 0; i<str.length();i++){
			arr[str.charAt(i)-'a']++;
		}
		for(int i = 25;i>=0;i--){
			if(arr[i]>0){
				for(int j = 0;j<arr[i];j++){
					System.out.print((char)('a'+(char)i));
				}
				break;
			}
		}
		log.close();
		
	}
	private static void permutation(String prefix, String str) {
	    int n = str.length();
	    if (n == 0) hs.add(prefix);
	    else {
	        for (int i = 0; i < n; i++)
	            permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i+1, n));
	    }
	}
	public static char give(char c1,char c2){
		if(c1!='a' && c2!='a') return 'a';
		if(c1!='b' && c2!='b') return 'b';
		return 'c';
	}
	static class Graph{
		int vertex;
		long weight;
	}
	public static void buildTree(int n){
		int arr[] = sc.nextIntArray(n);
		for(int i = 0;i<n;i++){
			int x = arr[i]-1;
			amp[i+1].add(x);
			amp[x].add(i+1);
		}
	}
	public static void bfs(int x,long arr[]){
		
		b[x] = true;
		q.add(x);
		while(!q.isEmpty()){
			int y = q.poll();
			for(int i = 0;i<amp[y].size();i++) {
				if(!b[amp[y].get(i)]){
					q.add(amp[y].get(i));
					b[amp[y].get(i)] = true;
				}
			}
		}
	}
		
	public static void buildGraph(int n){
		
		for(int i =0;i<n;i++){
			int x = sc.nextInt(), y = sc.nextInt();
			amp[--x].add(--y);
			amp[y].add(x);
		}
	}
		
	public static void dfs(int x){
		b[x] = true;
		for(int i =0 ;i<amp[x].size();i++){
			if(!b[amp[x].get(i)]){
				dfs(amp[x].get(i));
			}
		}
		
	}
	static class Pair implements Comparable<Pair> {
		int u;
		int v;
		int z;
		public Pair(){
			
		}
		public Pair(int u, int v) {
			this.u = u;
			this.v = v;
		}
		
		public int hashCode() {
			int hu = (int) (u ^ (u >>> 32));
			int hv = (int) (v ^ (v >>> 32));
			return 31*hu + hv;
		}

		public boolean equals(Object o) {
			Pair other = (Pair) o;
			return (u == other.u && v == other.v);
		}

		public int compareTo(Pair other) {
			return Integer.compare(Math.abs(u), Math.abs(other.u)) != 0 ? (Integer.compare(Math.abs(u), Math.abs(other.u))) : (Integer.compare(Math.abs(v), Math.abs(other.v)));
		}
		
		public String toString() {
			return "[u=" + u + ", v=" + v + "]";
		}
	}
	static class SegmentTree{
		int st[];
		SegmentTree(int arr[], int n){
			int size = 4*n+1;
			st = new int[size];
			build(arr,0,n-1,0);
		}
		int build(int arr[], int ss, int se, int si){
			if(ss==se){
				st[si] = arr[ss];
				return st[si];
			}
			int mid = (ss+se)/2;
			st[si] = build(arr,ss,mid,si*2+1)^build(arr,mid+1,se,si*2+2);
			return st[si];
		}
		int getXor(int qs, int qe, int ss, int se, int si){
			if(qe<ss || qs>se){
				return 0;
			}
			if(qs<=ss && qe>=se) return st[si];
			int mid = (ss+se)/2;
            return getXor(qs,qe,ss,mid,2*si+1)^getXor(qs,qe, mid+1, se, 2*si+2);
		}
		void print(){
			for(int i = 0;i<st.length;i++){
				System.out.print(st[i]+" ");
				System.out.println("----------------------");
			}
		}
	}
	static int convert(int x){
		int cnt = 0;
		String str = Integer.toBinaryString(x);
		//System.out.println(str);
		for(int i = 0;i<str.length();i++){
			if(str.charAt(i)=='1'){
				cnt++;
			}
		}
		int ans = (int) Math.pow(3, 6-cnt);
		return ans;
	}
	static class Node2{
		Node2 left = null;
		Node2 right = null;
		Node2 parent = null;
		int data;
	}
	static class BinarySearchTree{
		Node2 root = null;
		int height = 0;
		int max = 0;
		int cnt = 1;
		ArrayList<Integer> parent = new ArrayList<>();
		HashMap<Integer, Integer> hm = new HashMap<>();
		public void insert(int x){
			Node2 n = new Node2();
			n.data = x;
			if(root==null){
				root = n;
			}
			else{
				Node2 temp = root,temp2 = null;
				while(temp!=null){
					temp2 = temp;
					if(x>temp.data) temp = temp.right;
					else temp = temp.left;
				}
				if(x>temp2.data) temp2.right = n;
				else temp2.left = n;
				n.parent = temp2;
				parent.add(temp2.data);
			}
		}
		public Node2 getSomething(int x, int y, Node2 n){
			if(n.data==x || n.data==y) return n;
			else if(n.data>x && n.data<y) return n;
			else if(n.data<x && n.data<y) return getSomething(x,y,n.right);
			else return getSomething(x,y,n.left);
		}
		public Node2 search(int x,Node2 n){
			if(x==n.data){
				max = Math.max(max, n.data);
				return n;
			}
			if(x>n.data){
				max = Math.max(max, n.data);
				return search(x,n.right);
			}
			else{
				max = Math.max(max, n.data);
				return search(x,n.left);
			}
		}
		public int getHeight(Node2 n){
			if(n==null) return 0;
			height = 1+ Math.max(getHeight(n.left), getHeight(n.right));
			return height;
		}
	}
	static long findDiff(long[] arr, long[] brr, int m){
		int i = 0, j = 0;
		long fa = 1000000000000L;
		while(i<m && j<m){
			long x = arr[i]-brr[j];
			if(x>=0){
				if(x<fa) fa = x;
				j++;
			}
			else{
				if((-x)<fa) fa = -x;
				i++;
			}
		}
		return fa;
	}
	public static long max(long x, long y, long z){
		if(x>=y && x>=z) return x;
		if(y>=x && y>=z) return y;
		return z;
	}
	
	public static void seive(long n){
		b = new boolean[(int) (n+1)];
		Arrays.fill(b, true);
		for(int i = 2;i*i<=n;i++){
			if(b[i]){
				for(int p = 2*i;p<=n;p+=i){
					b[p] = false;
				}
			}
		}
	}
	static long modInverse(long a, long mOD2){
	            return  power(a, mOD2-2, mOD2);
	}
	static long power(long x, long y, long m)
	{
	    if (y == 0)
	        return 1;
	    long p = power(x, y/2, m) % m;
	    p = (p * p) % m;
	 
	    return (y%2 == 0)? p : (x * p) % m;
	}
	static long power2(long x,BigInteger y,long m){
		long ans = 1;
		BigInteger two = new BigInteger("2");
		while(y.compareTo(BigInteger.ZERO)>0){
			if(y.getLowestSetBit()==y.bitCount()){
				x = (x*x)%MOD;
				y = y.divide(two);
			}
			else{
				ans = (ans*x)%MOD;
				y = y.subtract(BigInteger.ONE);
			}
		}
		return ans;
	}
	static BigInteger power2(BigInteger x, BigInteger y, BigInteger m){
		BigInteger ans = new BigInteger("1");
		BigInteger one  = new BigInteger("1");
		BigInteger two  = new BigInteger("2");
		BigInteger zero  = new BigInteger("0");
		while(y.compareTo(zero)>0){
			//System.out.println(y);
			if(y.mod(two).equals(one)){
				ans = ans.multiply(x).mod(m);
				y = y.subtract(one);
			}
			else{
				x = x.multiply(x).mod(m);
				y = y.divide(two);
			}
		}
		return ans;
	}
	static BigInteger power(BigInteger x, BigInteger y, BigInteger m)
	{
	    if (y.equals(0))
	        return (new BigInteger("1"));
	    BigInteger p = power(x, y.divide(new BigInteger("2")), m).mod(m);
	    p = (p.multiply(p)).mod(m);
	 
	    return (y.mod(new BigInteger("2")).equals(0))? p : (p.multiply(x)).mod(m);
	}
	static long d,x,y;
	public static void extendedEuclidian(long a, long b){
		if(b == 0) {
	        d = a;
	        x = 1;
	        y = 0;
	    }
	    else {
	        extendedEuclidian(b, a%b);
	        int temp = (int) x;
	        x = y;
	        y = temp - (a/b)*y;
	    }
	}
	
	public static long gcd(long n, long m){
		if(m!=0) return gcd(m,n%m);
		else return n;
	}
	
	
	public static class FasterScanner {
		private byte[] buf = new byte[1024];
		private int curChar;
		private int numChars;
 
		public int read() {
			if (numChars == -1)
				throw new InputMismatchException();
			if (curChar >= numChars) {
				curChar = 0;
				try {
					numChars = System.in.read(buf);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (numChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}
 
		public String nextLine() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isEndOfLine(c));
			return res.toString();
		}
 
		public String nextString() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}
 
		public long nextLong() {
			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 int nextInt() {
			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 int[] nextIntArray(int n) {
	        return nextIntArray(n, 0);
	    }
	    
	    public int[] nextIntArray(int n, int off) {
	    	int[] arr = new int[n + off];
	    	for (int i = 0; i < n; i++) {
	    		arr[i + off] = nextInt();
	    	}
	    	return arr;
	    }
	    
	    public long[] nextLongArray(int n) {
	    	return nextLongArray(n, 0);
	    }
        
		public long[] nextLongArray(int n, int off) {
		    long[] arr = new long[n + off];
		    for (int i = 0; i < n; i++) {
		        arr[i + off] = nextLong();
		    }
		    return arr;
		}
 
	    private boolean isSpaceChar(int c) {
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}
 
		private boolean isEndOfLine(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}
	}
}