#include <cstdio>
#include <cassert>
#include <cmath>
#include <cstring>

#include <iostream>
#include <algorithm>
#include <vector>
#include <map>
#include <set>
#include <functional>
#include <stack>
#include <queue>

#include <tuple>

#define getchar getchar_unlocked
#define putchar putchar_unlocked

#define _rep(_1, _2, _3, _4, name, ...) name
#define rep2(i, n) rep3(i, 0, n)
#define rep3(i, a, b) rep4(i, a, b, 1)
#define rep4(i, a, b, c) for (int i = int(a); i < int(b); i += int(c))
#define rep(...) _rep(__VA_ARGS__, rep4, rep3, rep2, _)(__VA_ARGS__)

using namespace std;

using i64 = long long;
using u8 = unsigned char;
using u32 = unsigned;
using u64 = unsigned long long;
using f80 = long double;

int get_int() {
  int c, n;
  while ((c = getchar()) < '0');
  n = c - '0';
  while ((c = getchar()) >= '0') n = n * 10 + (c - '0');
  return n;
}

void solve() {
  int N, K;
  while (~scanf("%d %d", &N, &K)) {
    char str[1000010]; scanf("%s", str);
    const int l = strlen(str);
    copy(str, str + l, str + l);
    double lo = 0.0, hi = 1.0;

    rep(_, 50) {
      double mid = (lo + hi) / 2.;
      double s = 0.0, best = 0, t = 0.0;
      rep(i, K) s += str[i] - '0' - mid;
      bool ok = false;
      rep(i, K, 2 * l) {
        if (s > best) {
          ok = true;
          break;
        }
        s += str[i] - '0' - mid;
        t += str[i - K] - '0' - mid;
        best = min(best, t);
      }
      if (ok) lo = mid;
      else hi = mid;
    }
    printf("%.10f\n", lo);
  }
}

int main() {
  clock_t beg = clock();
  solve();
  clock_t end = clock();
  fprintf(stderr, "%.3f sec\n", double(end - beg) / CLOCKS_PER_SEC);
  return 0;
}