class Program
{
    static void Main(string[] args)
    {
        string input = Console.ReadLine()!;
        string word = "abcdefghijklmnopqrstuvwxyz";

        for (int i = 0; i < input.Length; i++)
        {
            if (input[i] != word[i])
            {
                Console.WriteLine(word[i] + "to" + input[i]);
                return;
            }
        }
    }
}