#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>

typedef long long ll;

const ll mod=1e9+7;

const int dx[] = {0, 1, 0, -1};
const int dy[] = {1, 0, -1, 0};

#define all(x) x.begin(),x.end()
#define allr(x) x.rbegin(),x.rend()
#define rc(x)  return cout<<x<<"\n",0
#define sz(s)  (int) s.size()
#define pb push_back
#define mp make_pair
#define fr first
#define sc second

using namespace std;

ll t,s[28],c,n,k;
vector<double>prob;
vector<double>ans;
	
void scr() {
	double a=1;
	for (int i=1; i<=k; i++) {
		a*=prob[s[i]-1];
	}
	ans.pb(a);
}
int perm(int b) {
	if (b==n) {
		c++;
		scr();
	}
	else {
		for (int r=b; r<=n; r++) {
			swap(s[b],s[r]);
			
			perm(b+1);
			
			swap(s[b],s[r]);
		}
	}
}

int main() {
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
	ifstream cin("dezastru.in");
	ofstream cout("dezastru.out");
	cin >> n >> k;
	for (int i=0; i<n; i++) {
		double x;
		cin >> x;	
		prob.pb(x);
	}
	for (int i=1; i<=n; i++) {
		s[i]=i;
	}
	perm(1);
	double rasp=0;
	for (int i=0; i<c; i++) {
		rasp+=(ans[i]/c);
	}
	cout << rasp << '\n';
}