#include<stdio.h>
#include<string.h>
int main() {
    char s[20];
    int freq[50]={0};
    int i,len,found=0;
    scanf("%s",s);
    len=strlen(s);
    for(i=0;i<len;i++){
        if(freq[(unsigned char)s[i]] == 1){
            printf("%c",s[i]);
            found=1;
            break;
        }
    }
    if(!found)
    printf("%c");
    return 0;
}