#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
typedef double f64;
const int N = 1e6 + 10;
const f64 eps = 1e-7;
int n, k;
string s;
f64 a[N], sum[N];
inline bool check(f64 x)
{
    for (int i = 1; i <= n; i++)
        a[i] = a[i + n] = (s[i] - '0' - x);
    for (int i = 1; i <= 2 * n; i++)
        sum[i] = sum[i - 1] + a[i];
    deque<int> q;
    q.push_back(0);
    for (int i = k; i <= 2 * n; i++)
    {
        while (!q.empty() && i - q.front() > n)
            q.pop_front();
        int j = q.front();
        if (sum[i] - sum[j] > eps)
            return 1;
        while (!q.empty() && sum[q.back()] - sum[i - k + 1] > eps)
            q.pop_back();
        q.push_back(i - k + 1);
    }
    return 0;
}
int main(int argc, char const *argv[])
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr), cout.tie(nullptr);
    cin >> n >> k >> s;
    s = ' ' + s;
    f64 l = 0, r = 1, ans = l;
    while (r - l > eps)
    {
        f64 mid = (l + r) / 2;
        if (check(mid))
            ans = mid, l = mid;
        else
            r = mid;
    }
    cout << fixed << setprecision(15) << ans << endl;
    return 0;
}