#include <fstream>
#include <iomanip>

using namespace std;

ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n,k,nr;
double v[27],sum;
void bk(int x,double s, int p)
{
    if(x>k)
    {
        sum += s;
        nr++;
    }
    else
    {
        for(int i = p+1; i <= n-k+x; ++i)
        {
            bk(x+1,s*v[i],i);
        }
    }
}
int main()
{
    f>>n>>k;
    for(int i=1;i<=n;i++)
        f>>v[i];
    bk(1,1,0);
    g<<fixed<<setprecision(6)<<sum/nr;
    return 0;
}