#include <bits/stdc++.h>
using namespace std;

const int MAXN = int(5e5);

int N, K;
string A;

double B[MAXN << 1];
double C[MAXN * 2 + 1];

bool ok(double avg) {
  for (int i = 0; i < N; ++i) {
    B[N + i] = B[i] = A[i] - '0' - avg;
  }
  for (int i = 0; i < N << 1; ++i) {
    C[i + 1] = C[i] + B[i];
  }
  deque<pair<double, int>> dq;
  for (int i = K; i < N; ++i) {
    while (!dq.empty() && dq.back().first <= C[i]) dq.pop_back();
    dq.emplace_back(C[i], i);
  }
  for (int i = 0; i < N; ++i) {
    while (!dq.empty() && dq.front().second - i < K) dq.pop_front();
    while (!dq.empty() && dq.back().first <= C[i + N]) dq.pop_back();
    dq.emplace_back(C[i + N], i + N);
    if (dq.front().first - C[i] >= 0.0) return true;
  }
  return false;
}

signed main() {
  ios::sync_with_stdio(false);
  cin >> N >> K;
  cin >> A;
  double lb = 0.0, ub = 1.0;
  for (int i = 0; i < 30; ++i) {
    double mb = (lb + ub) / 2;
    (ok(mb) ? lb : ub) = mb;
  }
  cout << fixed << setprecision(9) << lb << endl;
  return 0;
}