#include <stdio.h>
#include <stdlib.h>

int main() {
    int n, k;
    scanf("%d %d", &n, &k);
    
    int nums[100005] = {0};
    for (int i = 0; i < n; ++i) {
        scanf("%d", &nums[i]);
    }
    
    // 计算第一个长度为 k 的子数组的和
    int sum = 0;
    for (int i = 0; i < k; ++i) {
        sum += nums[i];
    }
    int maxSum = sum;
    
    // 滑动窗口
    for (int i = k; i < n; ++i) {
        sum += nums[i] - nums[i - k];
        if (sum > maxSum) {
            maxSum = sum;
        }
    }
    
    double maxAvg = (double)maxSum / k;
    printf("%.5f\n", maxAvg);
    return 0;
}