#include <iostream>
#include <cstdio>
#include <algorithm>
#include <string>

using namespace std;

int main()
{
    string s;
    getline(cin , s);
    for(int i = 0;i < s.size();i ++)
    {
        int count = 0;
        for(int j = 0;j < s.size();j ++)
        {
            if(s[j] == s[i])
            {
                count ++;
            }
            if(count > 1)
            {
                break;
            }
        }
        if(count == 1)
        {
            cout << s[i] << endl;
            return 0;
        }
    }
    cout << "no" << endl;
    return 0;
}