#include <iostream>
using namespace std;
int main() {
    string s; cin >> s;
    char alphabet = 'a';
    for(int i = 0; i < 26; ++i) {
        if (alphabet != s.at(i)) {
            cout << alphabet << "to" << s.at(i) << endl;
            break;
        }
        alphabet++;
    }
}