#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++; }
	else for(int i=ia+1; i<=n-(k-k1); 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/=t;
	printf("%lf\n",r);
	return 0;
}