#include<cstdio>
using namespace std;
int n,m,i,a[30],uz[30];
double p[30],prod;
long long fact=1;
void calc()
{int i;double mul=1;
	for(i=1;i<=m;i++)
		mul*=p[a[i]];
	prod+=mul;
}
void aranj(int k)
{
	if(k==m+1)calc();
	for(int i=a[k-1]+1;i<=n;i++)
	if(!uz[i])
	{
	  a[k]=i;
	  uz[i]=1;
	  aranj(k+1);
	  uz[i]=0;
	}
}
int main()
{
	freopen("dezastru.in","r",stdin);freopen("dezastru.out","w",stdout);
	scanf("%d %d",&n,&m);
	for(i=m+1;i<=n;i++)fact*=i;
	for(i=1;i<=n;i++) scanf("%lf",&p[i]);
	aranj(1);
	printf("%lf\n",prod/fact);
return 0;}