#include <fstream>
#include <iomanip>
using namespace std;
ifstream in("dezastru.in");
ofstream out("dezastru.out");
int n,m,a[26];
double p,b[26],s,nr;
void back(int k){
	for(int i=a[k-1]+1;i<=n;i++){
			a[k]=i;
			p=p*b[i];
			if(k!=m)
				back(k+1);
			else{
			   s=s+p;
			   nr++;}
		p=p/b[i];}}
int main(){
    in>>n>>m;
    for(int i=1;i<=n;i++)
		in>>b[i];
	p=1;
	back(1);
	out<<setprecision(6)<<s/nr;
    return 0;}