using System;

namespace _8._LongestBlockInString
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            int counter = 0;
            int maxCounter = 0;
            string result = string.Empty;

            for (int i = 1; i <= input.Length -1; i++)
            {
                if (result.Length >= input.Length - i)
                {
                    break;
                }
                if (input[i-1] == input[i] )
                {
                    counter++;

                    if (counter >= maxCounter)
                    {
                        maxCounter = counter;
                        result = new string(input[i], maxCounter + 1);
                    }
                }
                else
                {
                    counter = 0;
                }
            }
                Console.WriteLine(result);
        }
    }
}