#include <iostream>
#include <cstring>

using namespace std;

int main (){
    string str;
    getline(cin, str);

    int a[26] = {0};

    for (int i = 0; i < str.size(); i++)    a[str[i] - 'a'] ++;

    for (int i = 0; i < str.size(); i++)
        if (a[str[i] - 'a'] == 1) { // 这里不是 a[i] == 1
            cout << str[i] << endl;
            return 0;
        }   

    puts("no");
    return 0;
}