#include<stdio.h>
#include<string.h>
int main(){
    char s[101];
    scanf("%[^\n]", s);
    int freq[256] = {0};
    for (int i=0;s[i] != '\0';i++){
    freq[(unsigned char)s[i]]++;
}
 for (int i=0;s[i] != '\0';i++){
 if freq[(unsigned char)s[i]] == 1) {
 printf("%c", s[i]);
 return 0;
 }
 }
 printf("-1");
 return 0;
 }