#include<fstream>
using namespace std;
int n,k1,x[30],nrc;
double p[30],probfinal;

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

inline void back(int k,double prob)
{
	if(k>k1)
	{
		
		probfinal=probfinal+prob;
		++nrc;
	}
	else
	for(int i=x[k-1]+1;i<=n-k1+k;++i)
	{
		x[k]=i;
			back(k+1,prob*p[i]);
	}
}

int main()
{
	
	in>>n>>k1;
	for(int i=1;i<=n;++i)
		in>>p[i];
	back(1,1.0);
	out<<(1.0*probfinal)/(1.0*nrc);
	return 0;
}