#include <stdbool.h>
#include <stdio.h>
#include <string.h>

int main() {
    char s[1001] = {};
    char c;
    int length = 0;
    int sum = 0;
    bool flag = false;

    scanf("%s", s);
    length = strlen(s);
    for (int i=0; i<length; ++i) {
        sum = 0;
        for (int j=0; j<length; ++j) {
            if (s[i] == s[j]) sum++;
            else {}
        }
        if (sum == 1) {
            printf("%c", s[i]);
            flag = true;
            break;
        } else {}
    }

    if (!flag) {
        printf("-1");
    }
    
    return 0;
}