#include <iostream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <set>
#include <vector>
#include <map>
#include <queue>
#include <stack>
#include <utility>
#include <string>
#include <cstring>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <limits>
#include <sstream>
#include <deque>
#include <bitset>
#include <complex>
#include <functional>
#include <memory>
#include <numeric>
#define x first
#define y second
typedef std::pair<int, int> pii;
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
long long n, x[100], m;
double a[100], s, sum = 0;
void back(int k)
{
	if(k - 1 == m)
	{
		s = 1.0;
		for(int i = 1; i <= m; i++)
			s *= a[x[i]];
		sum  += s;
	}
	else
		for(int i = x[k - 1] + 1; i <= n; i++)
		{
			x[k] = i;
			back(k + 1);
		}
}
long long fact(int mata) {
	if(!mata) return 1LL;
	return fact(mata - 1) * mata;
}

int main () {
	fin >> n >> m;
	for(int i = 1; i <= n; i++)
		fin >> a[i];
	back(1);
	fout.precision(6);
	fout.setf(ios::fixed);
	fout << sum * fact(m) * fact(n - m) / fact(n);
	return 0;
}