#include <fstream>
#include <iomanip>
using namespace std;

double p[30],Ar[30][30];
long long Co[30][30];
int n,k;

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

	f >> n >> k;
	for(int i=1;i<=n;i++)
		f >> p[i];
	for(int i=0;i<=n;i++)
		Co[i][0]=Ar[i][0]=1;

	for(int i=1;i<=n;i++){
		for(int j=1;j<=n;j++){
			Ar[i][j]=Ar[i-1][j]+Ar[i-1][j-1]*p[i];
			Co[i][j]=Co[i-1][j]+Co[i-1][j-1];
		}
	}
	
	g << fixed << setprecision(6) << Ar[n][k]/Co[n][k];
}