#include<bits/stdc++.h>

using namespace std;

typedef long long int64;

int main()
{
  int64 N, K;
  string A;

  cin >> N >> K;
  cin >> A;

  auto check = [&](double k)
  {
    int sum = 0;

    queue< double > lazy, lazy2;
    multiset< double > bit;
    bit.emplace(1e9);
    lazy.emplace(0.0);
    lazy2.emplace(0.0);
    for(int i = 0; i < N + N; i++) {
      if(K <= i + 1) {
        bit.emplace(lazy.front());
        lazy.pop();
      }
      if(K <= i + 1 - N) {
        bit.erase(bit.find(lazy2.front()));
        lazy2.pop();
      }
      sum += A[i % N] - '0';
      if(*bit.begin() <= sum - (i + 1) * k) return (true);
      lazy.emplace(sum - (i + 1) * k);
      lazy2.emplace(sum - (i + 1) * k);
    }
    return (false);
  };

  double low = 0.0, high = 1.0;
  for(int i = 0; i < 44; i++) {
    double mid = (low + high) * 0.5;
    if(check(mid)) low = mid;
    else high = mid;
  }
  cout << fixed << setprecision(10) << low << endl;

}