#include<iostream>
#include<string>
#include<vector>
using namespace std;
int main(){
	string s;
	cin >> s;
	vector<int> a(26, 0);
    for(auto x: s){
        a[x - 'a']++;
    }
    for(auto x: s){
        if(a[x - 'a'] == 1){
            cout << x;
            return 0;
        }
    }
    cout << "no";
	return 0;
}