#include<cstdio>
#define NMax 28
using namespace std;
int n,k;
float a[NMax];
double suma;
int numar;

void Back(int pas,int x,double p)
{
	if(pas==k)
	{
		suma+=p;
		numar++;
		return;
	}
	for(int i=x;i<=n-k+pas;i++)
		Back(pas+1,i+1,p*a[i]);
}

int main()
{
	int i;
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d %d",&n,&k);
	for(i=0;i<n;i++)
		scanf("%f",a+i);
	Back(0,0,1);
	printf("%.6lf\n",suma/numar);
	return 0;
}