#include <cstdio>
#define NMAX 30
int N,P;
float val[NMAX];
int e[NMAX];
float REZ=0;
int NR=0;

void citire()
{
	scanf("%d %d",&N,&P);
	for(int i=1;i<=N;i++)
		scanf("%f",&val[i]);
}

void calcul()
{
	float a=1;
	for(int i=1;i<=P;i++)
		a*=val[e[i]];

	REZ+=a;
	NR++;
}

void back(int k)
{
	if(k==P+1)
	{
		calcul();
		return;
	}
	for(int i=e[k-1]+1;i<=N;i++)
	{
		e[k]=i;
		back(k+1);
	}
}

int main()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	citire();
	back(1);
	printf("%f",REZ/NR);
	
	
	return 0;
}