#include<iostream>

using namespace std;

int cnt[26];    //如果定义在main函数中会错误

int main()
{
    string str;
    cin >> str;


    for (auto c: str ) cnt[c - 'a'] ++ ;

    for (auto c: str )
    {    if (cnt[c - 'a'] == 1)
         {
             cout << c << endl;
             return 0;
         }
    }

    cout << "no" << endl;

    return 0;
}