#include <iostream>
#include <string>

using namespace std;

int main() {
    string S;
    cin >> S;

    int count[26] = {0};
    for (char c : S) {
        count[c - 'A']++;
    }

    int maxCount = 0;
    for (int i = 0; i < 26; i++) {
        if (count[i] > maxCount) {
            maxCount = count[i];
        }
    }

    for (int i = 0; i < 26; i++)  {
        if (count[i] == maxCount) {
            cout << (char)(i + 'A');
        }
    }

    cout << endl;
    return 0;
}