#include <iostream>
#include <cstring>
using namespace std;

int main() {
    string s;
    bool is_contain = false;
    getline(cin, s);
    const int len = 26;
    int nlist[len] = {0};
    int k = s.length();

    for (int i = 0; i < k; i ++) nlist[s[i] - 'a'] ++;

    for (int i = 0; i < k; i++) {
        if (nlist[s[i] - 'a'] == 1) {
            cout << s[i];
            is_contain = true;
            break;
        }
    }

    if (is_contain == false) cout << "no";

    return 0;
}