#include <fstream>
#include <vector>
#include <algorithm>
#include <iomanip>
#define nmax 26

using namespace std;

ifstream in("dezastru.in");
ofstream out("dezastru.out");

vector<double> v;
double d[nmax];
int N, m, cnt;

double prod = 1, suma;

int main ()
{
	in >> N;
	in >> m;
	for(int i=1; i<=N; i++)
    {
        in>>d[i];
    }
	for (int i = 1; i <= N; ++i)
		v.push_back(d[i]);
	do {
		for (int i = 0; i < m; ++i)
			prod*=v[i];
        suma+=prod;
        prod = 1;
		cnt++;
	} while (next_permutation(v.begin(), v.end()));
	out<<setprecision(6)<<suma/cnt;
	return 0;
}