#include <iostream>
#include <fstream>
#include <iomanip>
 
using namespace std; 
int n,m;
double a[100][100], v[100];
long long nrp[100][100]; 
void prm()
{
    a[0][0] = 1;
	nrp[0][0] = 1; 
for ( int i = 1; i <= n; i++ )
{
	a[i][0] = 1;
	nrp[i][0] = 1;
}

for ( int i = 1; i <= n; i++ )
	for ( int j = 1; j <= m; j++ )
	{
	nrp[i][j] = nrp[i - 1][j] + nrp[i - 1][j - 1]; // calcul nr permutari
	a[i][j] = a[i - 1][j] + a[i - 1][j - 1] * v[i]; // calcul produs
	}
}
 
int main()
{
    ifstream f("dezastru.in");
    ofstream g("dezastru.out");
 
    f >> n >> m;
 
    for ( int i = 1; i <= n; i++ )
	f >> v[i];
	prm();
    g << a[n][m] / nrp[n][m] << setprecision( 6 );
	
    f.close();
    g.close();
 
    return 0;
}