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