#include <bits/stdc++.h>
using namespace std;
#define int              long long int
#define double           long double
#define print(a)         for(auto x : a) cout << x << " "; cout << endl


const int M = 1000000007;
const int N = 3e5+9;
const int INF = 2e9+1;
const int LINF = 2000000000000000001;

inline int power(int a, int b, int mod=M) {
    int x = 1;
    a %= mod;
    while (b) {
        if (b & 1) x = (x * a) % mod; 
        a = (a * a) % mod;
        b >>= 1;
    }
    return x;
}


//_ ***************************** START Below *******************************

// babcb


string a;

char consistency(int n){
	
	vector<int> f(26, 0);
	
	for(int i=0; i<n; i++){
		int val = (i+1) * (n-i);
		f[a[i]-'a'] += val;
	}
	
	int maxi = 0;
	char ch = 'a';
	
	for(int i=0; i<26; i++){
		if(f[i] > maxi){
			maxi = f[i];
			ch = char(i+'a');
		}
	}
	
	return ch;
}















char practice(int n){


}





void solve() {
    
    int n;
    
    cin >> a;
    n = a.size();
    
    cout << consistency(n) << endl;


}





int32_t main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    int t = 1;
    // cin >> t;
    while (t--) {
        solve();
    }

    return 0;
}