#include <iomanip>
#include <iostream>
#include <stdio.h>

using namespace std;

double a[128];

int main() {
    freopen("temperatures.in","r",stdin);
    freopen("temperatures.out","w",stdout);

    int n, k;
    double maxAv, minAv, sum = 0;
    cin >> n >> k;
    for (int i = 0; i < n; i++)  cin >> a[i];
    for (int i = 0; i < k; i++) sum += a[i];
    maxAv = minAv = sum / k;
    for (int i = k; i < n; i++) {
        sum += a[i] - a[i - k];
        if (maxAv < sum / k) maxAv = sum / k;
        if (minAv > sum / k) minAv = sum / k;
    }
    cout << fixed << setprecision(2) << maxAv - minAv << endl;
    return 0;
}