#include <fstream>

using namespace std;

int n, m, nrc, m1;
double a[28], sol;

inline void back(int k, int s, double sum)
{
	if (k == m1)
	{
		sol += sum;
		nrc++;
	}
	else
		for (int i=s + 1; i<=n-m+k; i++)
			back(k+1, i, sum*a[i]);
}

int main()
{
	ifstream f("dezastru.in");
	f>>n>>m;
	int i;
	for (i=1; i<=n; i++)
		f>>a[i];
	f.close();	
	
	m1 = m + 1;
	back(1, 0, 1);
	
	ofstream g("dezastru.out");
	g<<sol/nrc<<"\n";
	g.close();
	
	return 0;
}