#include<stdio.h>
int a[1000001];
main() {
	int j,i,n,m;
	scanf("%d %d",&n,&m);
	for(i = 0; i < n; i++) {
		scanf("%d",&a[i]);
	}
	double avg = 0;
	for(i = 0; i <= n - m; i++) {
		//求m个小时的最大值
		double s = 0;
		for(j = i; j < i + m ; j++) {
			s = s + a[j];
		}
		s = s / m;

		if(s > avg) {
			avg = s;
		}

	}

	printf("%.2f",avg);
}
/**************************************************************
	Problem: 1165
	User: admin
	Language: C
	Result: Accepted
	Time:11 ms
	Memory:5052 kb
****************************************************************/