#include <stdio.h>
#define N 30
int fact[N],t;
int sol[N],n,r;
double	rez,v[N];
void precalcul()
{
	fact[0]=1;
	int i;
	for (i=1; i<=25; i++)
		fact[i]=fact[i-1]*i;
}
void citire()
{
	scanf("%d%d",&n,&r);
	int i;
	for (i=1; i<=n; i++)
		scanf("%llf",&v[i]);
	t=fact[r]*fact[n-r];
}
void back(int k)
{
	double rez_part;
	if (k==r+1)
	{
		rez_part=1;
		for (int i=1; i<=k-1; i++)
			rez_part*=v[sol[i]];
		rez_part*=t;
		rez+=rez_part;
		return;
	}
	for (int i=sol[k-1]+1; i<=n; i++)
	{
		sol[k]=i;
		back(k+1);
	}
}
int main()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	precalcul();
	citire();
	back(1);
	rez/=fact[n];
	printf("%.6llf",rez);
	return 0;
}