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

int main(){
    char s[101];
    int freq[256] ={0};
    scanf("%s",s);
    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;
}