#include<stdio.h>
int n,k,t=0,ia=0;
double v[30],r=0;
void back(int k1,int ia,double par)
{
	if(k1==k+1)
	{
		r+=par;
		t++;
		//printf("%f\n",par);
		return;
	}
	int i;
	for(i=ia+1; i<=n; i++)
		back(k1+1,i,par*v[i]);
}
int main()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d%d",&n,&k);
	int i;
	for(i=1; i<=n; i++)
		scanf("%lf",&v[i]);
	back(1,0,1);
	r=r/t;
	printf("%lf\n",r);
	//printf("%f\n",v[2]);
	return 0;
}