#include<stdio.h>

#define IN "dezastru.in","r",stdin
#define OUT "dezastru.out","w",stdout

int n,T;
double rez;
double prob[30],sol[30];
int marc[30];

void citire()
{
	freopen(IN);
	scanf("%d%d",&n,&T);
	for(int i=1;i<=n;++i)
		scanf("%lf",&prob[i]);
}
void back(int k)
{
	if(k==T+1)
	{
		//for(int i=1;i<k;++i)
		//	printf("%d ",sol[i]);
	//	printf("\n");
		double rez1=1;
		for(int i=1;i<k;++i)
				rez1*=sol[i];
		rez+=(rez1/6);
		return;
	}
	for(int i=1;i<=n;++i)
	{
		if(!marc[i])
		{
			marc[i]=1;
			sol[k]=prob[i];
			back(k+1);
			marc[i]=0;
		}
	}
}
void afisare()
{
	freopen(OUT);
	printf("%.6lf",rez);
}
int main()
{
	citire();
	back(1);
	afisare();
	return 0;
}