using System;
class Program
    {
        private static char[] gems;
        private static int num;
        private static double result = 0;

        static void Main(string[] args)
        {
            num = int.Parse(Console.ReadLine().Split(' ')[1]);

            string line = Console.ReadLine();
            int lineLength = line.Length;
            line += line;

            gems = line.ToCharArray();

            while (num <= lineLength)
            {
                for (int i = 0; i < lineLength; i++)
                {
                    int blueNum = GetBlueGem(i);

                    double ratio = (double)blueNum / (double)num;

                    if (ratio > result)
                    {
                        result = ratio;
                    }
                }
                num++;
            }

            Console.WriteLine(result);
        }

        private static int GetBlueGem(int index)
        {
            int ret = 0;

            for (int i = 0; i < num; i++)
            {
                if(gems[index + i] == '1')
                {
                    ret++;
                }
            }

            return ret;
        } 
    }