#include<bits/stdc++.h>

using namespace std;

typedef long long int64;

std::vector< int > slideMinimum(const std::vector< double > &a, int k)
{
  int n = a.size();
  std::deque< int > deq;
  std::vector< int > res;
  for(int i = 0; i < n; i++) {
    while(deq.size() && a[deq.back()] >= a[i]) deq.pop_back();
    deq.push_back(i);
    if(i - k + 1 >= 0) {
      res.push_back(deq.front());
      if(deq.front() == i - k + 1) deq.pop_front();
    }
  }
  return res;
}


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

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

  auto check = [&](double k)
  {
    int sum = 0;
    queue< double > lazy;
    lazy.emplace(0.0);
    vector< double > all(N + N + N, 1e9);
    int last = 11451419;
    for(int i = 0; i < N + N; i++) {
      if(K <= i + 1) {
        all[i + N - 1] = lazy.front();
        last = min(last, i);
        lazy.pop();
      }
      sum += A[i % N] - '0';
      lazy.emplace(sum - (i + 1) * k);
    }
    auto p = slideMinimum(all, N);

    sum = 0;
    for(int i = 0; i < N + N; i++) {
      sum += A[i % N] - '0';
      if(last <= i) {
        if(all[p[i]] <= sum - (i + 1) * k) return (true);
      }
    }
    return (false);
  };

  double low = 0.0, high = 1.0;
  for(int i = 0; i < 23; i++) {
    double mid = (low + high) * 0.5;
    if(check(mid)) low = mid;
    else high = mid;
  }
  cout << fixed << setprecision(10) << low << endl;

}