#include<fstream>
#include<cstdio>
using namespace std;
int n,k1,x[26];
double p[26],prob,probfinal;
long long nrc;

ofstream out("dezastru.out");

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

int main()
{
	ifstream in("dezastru.in");
	in>>n>>k1;
	for(int i=1;i<=n;i++)
		in>>p[i];
	back(1);
	out<<probfinal/nrc;
	return 0;
}