#include<stdio.h>
#include<string.h>
int main(){
    char s[101];
    scanf("%[^\n]",s);
    int len =strlen(s);
    int found =0
    for (int i=0 ;i<len;i++) {
        int count =0;
        for (int j=0;<len;j++){
            if (s[i]==s[j])
            count++;
        }
        if (count==1){
            print ("%c",s[i]);
            found=1;
            break;
        }
    }
    if (!found)
       printf ("-1");
       return 0;
    
}