//复习完毕
#include <iostream>

using namespace std;

int cnt[26];

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

    for (auto x : a) cnt[x - 'a'] ++ ;
    for (auto x : a)
        if (cnt[x - 'a'] == 1)
        {
            cout << x << endl;
            return 0;
        }
    puts("no");

    return 0;
}