#include <fstream>
#include <iomanip>
#define NMAX 26 * 26

using namespace std;

ifstream f("dezastru.in");
ofstream g("dezastru.out");
int comb=0;
int n,k;
double sum=0,v[NMAX];
void BACKTRACK(int val,int p,double dezAct)
{
    if(p==k)
    {
        sum+=dezAct;
        comb++;
        return;
    }
    for(int i=val+1; i<=n-(k-p)+1; i++)
        BACKTRACK(i,p+1,dezAct*v[i]);
}
int main()
{
    f>>n>>k;
    for(int i=1; i<=n; i++)
        f>>v[i];
    BACKTRACK(0,0,1);
    g<<fixed<<setprecision(7)<<(sum/comb);
}