class Program
    {
        static void Main(string[] args)
        {
            string sentence = Console.ReadLine();

            Dictionary<string, int> result = new Dictionary<string, int>();

            for(int i = 0; i < sentence.Count() - 1; i++)
            {
                char current = sentence.ElementAt(i);
                char next = sentence.ElementAt(i + 1);

                if(current == next)
                {
                    string pair = current.ToString() + next.ToString();
                    if(result.ContainsKey(pair))
                    {
                        result[pair]++;
                    } else
                    {
                        result.Add(pair, 1);
                    }
                }
            }

            var value = result.Values.Max();
            var key = result.Keys.Where(x => result[x] == value && x != " ").Min();

            Console.WriteLine(key);
            Console.ReadLine();
        }
    }