#include <iostream>
#include <string>
using namespace std;

int g[1005];


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

    for (auto c : s) {
        g[c] ++;
    }

    int flag = 0;
    for (auto c : s) {
        if (g[c] == 1) {
            flag = 1;
            cout << c;
            break;
        }
    }
    if (!flag) cout << -1;
}