#include <iostream>
using namespace std;

#define U 500010
double A[U + U];
double Acum[U + U];
int deq[U + U];
int N, K;

bool test(double x)
{
    Acum[0] = 0;
    for (int i = 0; i < N + N; i++)
    {
        Acum[i + 1] = Acum[i] + A[i] - x;
    }
    int l = 0;
    int r = 0;
    deq[0] = 0;
    for (int k = K; k <= N + N; k++)
    {
        while (deq[l] < k - N)
        {
            l++;
        }
        double x = Acum[k];
        if (x > Acum[deq[l]])
        {
            return true;
        }
        int i = k - K + 1;
        x = Acum[i];
        while (l <= r && Acum[deq[r]] > x)
        {
            r--;
        }
        r++;
        deq[r] = i;
    }
    return false;
}

int main()
{
    cin >> N >> K;
    for (int i = 0; i < N; i++)
    {
        char c;
        cin >> c;
        A[i] = c - 48.0;
        A[i + N] = c - 48.0;
    }
    double l = 0.0;
    double r = 1.0;
    for (int i = 0; i < 20; i++)
    {
        double m = (l + r) / 2;
        if (test(m))
        {
            l = m;
        }
        else
        {
            r = m;
        }
    }
    cout << l << endl;
    return 0;
}