#include <iostream>
#include <cstring>
using namespace std;
const int N = 100010;
char str[N] = {0};

int main()
{
    int cnt[26] = {0};
    fgets(str, 100000, stdin);
    for (int i = 0; str[i]; i ++ )
    {
        if (str[i] > 'a' && str[i] < 'z')
        cnt[str[i] - 'a'] ++;
    }
    for (int i = 0; str[i]; i ++ )
    {
        if (cnt[str[i] - 'a'] == 1)
        {
            printf("%c", str[i]);
            return 0;
        }
    }
    printf("no");

    return 0;
}