using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[] NMi => ReadLine().Split().Select(c => int.Parse(c) - 1).ToArray();
    static int[][] NMap(int n) => Enumerable.Repeat(0, n).Select(_ => NMi).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var s = ReadLine();
        var ans = "abcdefghijklmnopqrstuvwxyz";
        for (var i = 0; i < s.Length; ++i)
        {
            if (s[i] != ans[i])
            {
                WriteLine($"{ans[i]}to{s[i]}");
                return;
            }
        }
    }
}