#include<stdio.h>
#include<string.h>
int main(){
    char s[105];
    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;
}