#include <iostream>
#include <fstream>
using namespace std;
int permutari[26],n,k,total;
double d,dez[26];
void rez(int ajuns,int numar,double produs);
void continua(int ajuns,int numar,double produs)
{
    register int i;
    static int verific[26];
    bool inceput=0;
    if (inceput==0)
    {
        inceput=1;
        for (i=1;i<=25;i++)
        {
            verific[i]=0;
        }
    }
    verific[permutari[numar-1]]=1;
    for (i=ajuns+1;i<=n;i++)
    {
        if (verific[i]==0)
        {
            permutari[numar]=i;
            rez(i,numar+1,produs*dez[i]);
        }
    }
    verific[permutari[numar-1]]=0;
}
void rez(int ajuns,int numar,double produs)
{
    if (ajuns<n&&numar<=k&&n-ajuns>=k-numar)
    {
        continua(ajuns,numar,produs);
    }
    else if (numar==k+1)
    {
        total++;
        d+=produs;
    }
}
int main()
{
    ifstream in("dezastru.in");
    ofstream out("dezastru.out");
    int i,j;
    in>>n;
    in>>k;
    for (i=1;i<=n;i++)
    {
        in>>dez[i];
    }
    rez(0,1,1);
    d/=total;
    out<<d;
}