#include <stdio.h>
#define MAXN 32

double p[MAXN],sum;
int n,k,c;

inline void backrec(int nivel,int st,double val)
{
	int i;
	if (nivel > k)
	{
		sum+=val;
		c++;
		return;
	}
	else
		for (i=st;i<=n-k+nivel;i++)
			backrec(nivel+1,i+1,val*p[i]);
}

int main()
{
	int i;
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d%d",&n,&k);
	for (i=1;i<=n;i++)
		scanf("%lf",&p[i]);

	backrec(1,1,1);

	printf("%.06lf",sum/c);

	return 0;
}