#include<iostream>
#include <iomanip>

int main() {
    int N, M;
    std::cin >> N >> M;

    int speeds[100];
    for (int i = 0; i < N; ++i) {
        std::cin >> speeds[i];
    }
    int windowSum = 0;
    for (int i = 0; i < M; ++i) {
        windowSum += speeds[i];
    }

    int maxSum = windowSum;
    for (int i = M; i < N; ++i) {
        windowSum = windowSum - speeds[i - M] + speeds[i];
        if (windowSum > maxSum) {
            maxSum = windowSum;
        }
    }

    double averageSpeed = static_cast<double>(maxSum) / M;
    std::cout << std::fixed << std::setprecision(2) << averageSpeed << std::endl;

    return 0;
}    
/**************************************************************
	Problem: 1165
	User: panyuchen
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/