#include<iostream>

using namespace std;

int cnt[26];//存放a~z各出现的次数

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

   for(int i = 0;s[i];i ++)
        cnt[s[i] - 'a'] ++;

    for(int i = 0;s[i];i ++)
    {
        if(cnt[s[i] - 'a'] == 1)
        {
            cout << s[i] << endl;
            return 0;
        }
    }
    cout << "no" << endl;
    return 0;
}