#include <bits/stdc++.h>
using ll = long long;
using llu = long long unsigned;

using namespace std;

int main() {
    string S;
    cin >> S;

    char c = 'a';
    for (auto x : S) {
        if (c != x) { cout << c << "to" << x << endl; return 0; }
        c++;
    }
    return 0;
}