#include<iostream>
#include<string>
#include<cmath>
#include<algorithm>
#include<cstdio>
#include<cstring>
#include<unordered_map>
#include<climits>
#define x first
#define y second
using namespace std;
unordered_map<char,int> unmap;
string s;
int main(){
	cin>>s;
	string res;
	for(char c:s){
		unmap[c]++;
	}
	int maxx=INT_MIN;
	for(unordered_map<char,int>::iterator it=unmap.begin();it!=unmap.end();it++){
		maxx=max(it->y,maxx);
	}
	for(unordered_map<char,int>::iterator it=unmap.begin();it!=unmap.end();it++){
		if(it->y==maxx) res+=it->x;
	}
	reverse(res.begin(),res.end());
        sort(res.begin(),res.end());
	cout<<res;
}