#include<cstdio>
using namespace std;
int d1(int a)
{
	int s=0;
	while(a)
		a&=(a-1),s++;
	return s;
}
int main()
{
	freopen("dezastru.in","r",stdin);freopen("dezastru.out","w",stdout);
	int bin=1,lim,i,n,k,n1,n2,j;float v[26],val,s=0,var=1;
	scanf("%d%d",&n,&k);
	for(i=1;i<=n;i++)
		scanf("%f",&v[i]);
	lim=1<<n;
	j=k+1;
	for(i=1;i<=n-k;i++)
		if(j<=n)
			var*=(float)i/j++;
		else
			var*=i;
	for(;bin<lim;bin++)
		if(d1(bin)==k)
		{
			val=1;
			for(i=0;i<n;i++)
				if((bin>>i)&1)
					val*=v[i+1];
			s+=var*val;
		}
	printf("%f",s);
}