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