#include<cstdio>#include<iostream>#include<cstdlib>#include<cstring>#include<cmath>#include<algorithm>using namespace std;const int MAXN=50005;pair<int,int> p[MAXN];int a[MAXN], sum[MAXN];double C[MAXN];void init(){ C[0]=0; for(int i=1; i<MAXN; i++){ C[i]=C[i-1]+log(i*1.0); }}double calc(int n, int m){ return C[n]-C[n-m]-C[m];}int main(){ int n, m, x; init(); while(cin>>n>>m&&n+m){ for(int i=0; i<n; i++){ scanf("%d",&a[i]); sum[i+1]=sum[i]+a[i]; } double ans=0; for(int i=m; i<=n; i++){ double p=exp(calc(i-2,m-2)-calc(n,m)); ans+=p*(sum[n]-sum[i-1]-sum[n-i+1]); } printf("%.3fn",ans); } return 0;}
