#include <cstdio>
#define N 26
using namespace std;
double p[N], ans=0;
int l=0;
int main()
{
    freopen("dezastru.in", "r", stdin);
    freopen("dezastru.out", "w", stdout);
    int i, n, k, j;
    scanf("%d%d", &n, &k);
    for(i=1; i<=n; ++i)
        scanf("%lf", &p[i]);
 	for(i=1;i<=n-k+1;i++)
 	for(j=i+1;j<=i+k;j++){
 		ans+=p[i]*p[j];
 		l++;
	 }
	 printf("%lf ", ans/(l-1));
	 return 0;
}