#include<stdio.h>

int sol[32];
int n,m;
float v[32];
float q;

float calc()
{
	float w=1;
	for(int i=1;i<=m;++i)
	{
		w*=v[sol[i]];
	}
	
	return w;
}


bool verif(int x)
{
	int i;
	for(i=1;i<x;++i)
		if(sol[x]==sol[i])
			return 0;
	
	return 1;
}

int back(int k)
{
	int i;
	
	if(k==n+1)
	{
		q+=calc();
		return 0;
	}
	
	for(i=1;i<=n;++i)
	{
		sol[k]=i;
		if(verif(k))
			back(k+1);
			
	}
	return 0;
}

int main()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;++i)
	{
		scanf("%f",&v[i]);
	}
	
	back(1);
	
	printf("%f",q/6);
	return 0;
}