#include<bits/stdc++.h>
#define NMAX 50
#define int long long
using namespace std;

int n,K,s[NMAX];
double rs;
double a[NMAX];
int f;
int lvl;

void BT(int k, double p, int nr) {
	if (k==K+1) {
		f++; rs+=p; 	
		return;
	}
	
	for (int i=nr+1; i<=n-K+k; i++) {
		BT(k+1,p*a[i],i);
	}
}


int32_t main() {
	ifstream cin("dezastru.in");
	ofstream cout("dezastru.out");
	cin>>n>>K;
	for (int i=1; i<=n; i++) cin>>a[i];
	BT(1,1,0);
	cout<<fixed<<setprecision(10)<<rs/f;
	
	return 0;
}