#include <fstream>
using namespace std;

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

void Back(int p);
void Write(int p);
bool OK(int p);
int n, k, l, x[26], nrsol;
long double a[26], sumfinal;

int main()
{
	
	fin >> n >> k;
	for ( int i = 1; i <= n; i++)
		fin >> a[i];
		
	Back(1);
	fout<<sumfinal/nrsol;
	fin.close();
	fout.close();
}

void Back(int p)
{
	if ( p > n )
	{	
		Write(p-1);
		return;
	}
	for ( int i = 1; i <= n; ++i)
	{
		x[p] = i;
		if ( OK(p) )
			Back(p+1);
	}
}

void Write(int p)
{
	nrsol++;
	long double prod = 1;
	for ( int i = 1; i <= 2; ++i)
		prod*=a[x[i]];
	
	sumfinal+=prod;
}

bool OK(int p)
{
	for ( int i = 1; i < p; i++)
		if ( x[i] == x[p] )
			return false;
		return true;
}