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

int N, K;
int A[100];
vector<double> av;

void readInput()
{
    int i;

    cin >> N >> K;
    for (i = 0; i < N; i++)
    {
        cin >> A[i];
    }
}

int main()
{
    /*
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    cout.precision(2);
    */
    av.clear();
    #ifdef ONLINE_JUDGE
    freopen("temperatures.in", "r", stdin);
    freopen("temperatures.out", "w", stdout);
    #endif // ONLINE_JUDGE

    readInput();

    int i, j, sum;
    double curAv, ans;

    for (i = 0; i <= N - K; i++)
    {
        sum = 0;
        for (j = i; j < i + K; j++)
        {
            sum = sum + A[j];
        }
        curAv = double(sum) / double(K);
        av.push_back(curAv);
    }

    sort(av.begin(), av.end());

    ans = av.back() - av[0];

    printf("%.2f\n", ans);

    return 0;
}