#include <fstream>
#define rational 1.0
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n,m,poz[20],nr;
double v[20],sum=1,sol;
inline void recurenta(int k,double sum)
{
  if(k>m)
	{
		sol=sol+sum;
		++nr;
	}
	else
	for(int i=poz[k-1]+1;i<=n-m+k;++i)
	{
		poz[k]=i;
		recurenta(k+1,sum*v[i]);
	}
}
int main()
{
	f>>n>>m;
	for(int i=1;i<=n;++i)
		f>>v[i];
	recurenta(1,rational);
	g<<(1.0*sol)/(1.0*nr);
	return 0;
}