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

#define MAXN 26
typedef unsigned long long uint64;

int n,k;
double probs[MAXN];
double total;

inline int maximum(const int a, const int b)
{
	return a>b?a:b;
}

inline int minimum(const int a, const int b)
{
	return a<b?a:b;
}

void comb(const int cur, const int l, const double prod)
{
	if(l==k)
		total+=prod;
	else
		for(int i=cur; i<=n-k+l+1; ++i)
			comb(i+1,l+1,prod*probs[i]);
}

int main()
{
	fstream fin("dezastru.in", fstream::in);
	fstream fout("dezastru.out", fstream::out);
	
	fin>>n>>k;
	uint64 Cnk=1,prod=1;
	for(int i=maximum(n-k,k)+1; i<=n; ++i)
		Cnk*=i;
	for(int i=1; i<=minimum(n-k,k); ++i)
		prod*=i;
	Cnk/=prod;
	
	for(int i=1; i<=n; ++i)
		fin>>probs[i];
	
	for(int i=1; i<=n-k+1; ++i)
		comb(i+1,1,probs[i]);

	fout<<setiosflags(fstream::fixed)<<setprecision(6)<<total/((double)Cnk)<<" \n";
	fin.close();
	fout.close();
	return 0;
}