#include<iostream>
#include<fstream>
#include<algorithm>
#include<iomanip>
using namespace std;
const int m=30;

ifstream fin("dexastru.in");
ofstream fout("dezastru.out");

float x[m]; //primele 2
float d[m][m];
float a[m]; //0,3 0,5 0,8
short n,k;

int main()
{
	fin>>n>>k;
	for(int i=1;i<=n;++i)
		cin>>a[i];	
	
	x[1]=1;
	
	for(int i=2;i<=n;++i)
	x[i]=i*x[i-1];
	
	for(int i=0;i<=n;++i)
	d[i][0] = 1;
	
	for(int i=1;i<=n;++i)
		for(int j=1;j<=n;++j)
			d[i][j]=d[i-1][j-1]*a[i]+d[i-1][j];
		
	fout<<setprecision(6)<<d[n][k]/(x[n]/(x[k]*x[n-k]))<<"\n";
	
	fin.close();
	fout.close();
	return 0;
	
}