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

int main() {
    char s[101];
    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++);
    if (count[(unsigned char)s[i]] ==1) {
        printf("%c", s[i]);
        found = 1;
        break;
    }
}
if (!found)
printf("-1");

return 0;
}