#include <fstream>
#include <iomanip>

using namespace std;

double a[30],ans=0;
int k,n,nr=0;
bool seen[30];

void comb(double s,int x){
	if(x==k+1){
		ans+=s;
		nr++;
		return; 
	}
	for(int i=1;i<=n;i++){
		if(!seen[i]){
			seen[i]=1;
			comb(s*a[i],x+1);
		}
		seen[i]=0;
	}
};


int main(){

	ifstream f("dezastru.in");
	ofstream g("dezastru.out");

	f >> n >> k;
	for(int i=1;i<=n;i++){
		f >> a[i];
	}

	comb(1,1);
	ans/=nr;
	g << fixed << setprecision(6) << ans;
}