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

int main(){
    char s[100];
    int count[256] = {0};
    int i, len, found = 0;
    
    scanf("%s", s);
    len = strlen(s);
    
    for (i = 0; i < len; i++){
        count (unsigned char)s[i]]++;
    }
    for (i = 0; i < len; i++){
        of (count (unsigned char)s[i]] == 1){
            printf("%c", s[i]);
            found = 1;
            break;
        }
        
    }
    if (!found)
        printf("-1");
    return 0;
}