#include<stdio.h>
#include<string.h>
int main(){
    char s[10];
    int freq[256]={0};
    scanf("%s",s);
    int len=strlen(s);
    for(int i=0;i<len;i++)
    {
        freq[(unsigned char)s[i]]++;
    }
    int found=0;
    for(int i=0;i<len;i++){
        if(freq[(unsigned char)s[i]==1){
        printf("%c\n",s[i]);
        found=1;
        break;
    }
}
    if(found){
        printf("-1\n");
    return 0;
    }
}