#include <fstream>
#include <iomanip>

using namespace std;

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

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


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

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

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

}