#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 < n;i++){
        if(freq(unsigned char)s[i]] ==1){
            printf("%c\n",s[i]);
            found = 1;
            break;
        }
        if(!found){
            printf("-1\n");
        }
            return 0;
        }
}