#include<stdio.h>
#include<string.h>
int main(){
    char s[101];
    int freq[256]={0};
    int i;
    char result='_';
    scan("%s",s);
    for(i=0;s[i]!='\0';i++){
        freq[(unsigned char)s[i]]++;
    }
    for(i=0;s[i]!='\0';i++){
        if(freq[(unsigned char)s[i]]==1){
            result=s[i];
            break;
        }
    }
    if(result!='_'){
        printf("%c\n",result);
    }
    else{
        printf("_1\n");
    }
    return 0;
}