#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
const int NMAX=26;
int n,x;
double a[NMAX],val=1,total=0;
int sol[NMAX],b[NMAX],p=0;
inline void Read(){

    f>>n>>x;
    for(int i=1;i<=n;i++)
        f>>a[i];
}
inline void Back(int k)
{
    if(k==x+1)
    {
        p++;
        total+=val;
        return;
    }
    for(int i=sol[k-1]+1;n-i>=x-k;i++)
    {
        sol[k]=i;
        val*=a[i];
        Back(k+1);
        val/=a[i];
    }
}
int main()
{
    Read();
    Back(1);
    double sol=total/p;
    g<<setprecision(6)<<fixed<<sol;
    return 0;
}