// dezstru.cpp : Defines the entry point for the console application.
//

#include <stdio.h>
int n,k,val[30],cont=0;
float l,e[30],t=0;
float d=1;
void back( int a )
{
	int i;
	if(a==k+1)
	{
		t=t+d;
		cont++;
		return ;
	}
	for(i=val[a-1]+1;i<=n-k+a;i++)
	{
		val[a]=i;
		d=d*e[i];
		back(a+1);
		d=d/e[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("%f ",&e[i]);
	back(1);
	l=t/cont;
	printf("%f",l);
	return 0;
}