#include <bits/stdc++.h>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int N,k,x=0; 
vector<int> v;
vector<double> s;
double a[26],aux,pr=0;
 
int main () {
	fin >> N >>k;
	for(int i=0;i<N; ++i) fin>>a[i];
	for (int i = 1; i <= N; ++i)
		v.push_back(i);
	do {
		aux=1;
		x++;
		for(int i = 0; i<k; ++i) aux*=a[v[i]-1];
		s.push_back(aux);
	} while (next_permutation(v.begin(), v.end()));
	for(int i=0; i<s.size(); i++) pr+= s[i]/x;
	fout<<fixed<<setprecision(6)<<pr;
}