#include <cstdio>
#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <deque>

// blue[l,r)/(r-l
// blue[l,r)/(r-l) >= x
// blue[l,r] >= x(r-l)
// blue[r]-blue[l] >= xr - xl
// blue[r]-xr >= blue[l] >= xl
// r-l>=K

int main() {
  int n, k;
  std::cin >> n >> k;

  std::string s;
  std::cin >> s;
  s += s;

  std::vector<int> sum(n * 2 + 1);
  for (int i = 0; i < n * 2; i++) {
    sum[i + 1] = sum[i] + (s[i % n] == '1');
  }

  double ok = 0;
  double ng = 1;
  for (int ii = 0; ii < 30; ii++) {
    double mid = (ok + ng) / 2;

    std::deque<std::pair<double, int>> deq;
    for (int i = k; i < n; i++) {
      while (!deq.empty() && deq.back().first < sum[i] - mid * i) deq.pop_back();
      deq.emplace_back(sum[i] - i * mid, i);
    }

    bool found = false;
    for (int i = 0; i < n; i++) {
      while (!deq.empty() && deq.front().second < i + k) deq.pop_front();
      while (!deq.empty() && deq.back().first < sum[i + n] - mid * (i + n)) deq.pop_back();
      deq.emplace_back(sum[i + n] - mid * (i + n), i + n);
      if (sum[i] - i * mid <= deq.front().first) {
        found = true;
      }
    }
    
    (found ? ok : ng) = mid;
  }

  printf("%.20f\n", ok);
}