#include<fstream>
#include<iostream>
using namespace std;
double n,k,sol[26][26],a[26];


double combinari(){
	double comb = 1;	
	int i;
	for(i =k+1;i<=n;i++){
		comb=comb*i;
	}
	for(i=2;i<=n-k;i++){
		comb=comb/i;
	}
	return comb;
}
int main(){
	ifstream f("dezastru.in");
	ofstream g("dezastru.out");
	f>>n>>k;
	int i,j;
	for(i=1;i<=n;i++){
		f>>a[i];
	}
	for(i=1;i<=n;i++){
		for(j=1;j<=i && j<=k;j++){
			sol[i][j]=sol[i-1][j]+sol[i-1][j-1]*a[i];
		}
	}
	g<<sol[(int)n][(int)k]/combinari();
	return 0;
}