#include<fstream>
using namespace std;
short n,k,v[27],uz[27];
double a[27],suma;
ofstream fout("dezastru.out");

void Prelucrare()
{
	double p=1;
	short i;
	for(i=1;i<=k;i++)
		p=p*a[v[i]];
	suma+=p;
}

void Back(short pas)
{
	short i,limita;
	if(pas==k+1)
		Prelucrare();
	else
	{
		limita=n-k+pas;
		for(i=v[pas-1]+1;i<=limita;i++)
		{
			if(uz[i]==0)
			{
				v[pas]=i;
				uz[i]=1;
				Back(pas+1);
				uz[i]=0;
			}
		}
	}
}

int main()
{
	int i;
	ifstream fin("dezastru.in");
	fin>>n>>k;
	for(i=1;i<=n;i++)
		fin>>a[i];
	fin.close();
	Back(1);
	for(i=k+1;i<=n;i++)
		suma=suma/i;
	fout<<suma<<"\n";
	fout.close();
	return 0;
}