#include <stdio.h>
#define N 30
int n,r,sol[N];
double v[N],rez;
void citire()
{
	int i;
	scanf("%d%d",&n,&r);
	for (i=1; i<=n; i++)
		scanf("%llf",&v[i]);
}
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*=2;
		rez+=rez_part;
		return;
	}
	for (int i=sol[k-1]+1; i<=n; i++)
	{
		sol[k]=i;
		back(k+1);
	}
}
int fact(int k)
{
	int rez=1,i;
	for (i=1; i<=k; i++)
		rez*=i;
	return rez;
}
int main()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	citire();
	back(1);
	rez/=fact(n);
	printf("%.6llf",rez);
	return 0;
}