#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int i,j,n,k,st[35],prob=0;
double sum=0,v[35];
inline void bktr(int val,int p,double pr)
{
    st[p]=val;
    if(p==k)
        {
            sum=sum+pr;
            prob++;
            return;
        }
        int m=n-(k-p)+1;
        for(int i=val+1;i<=m;i++)
        {
                bktr(i,p+1,pr*v[i]);
        }
}
int main()
{
    f>>n>>k;
    for(i=1;i<=n;i++)
    {
        f>>v[i];
    }
    bktr(0,0,1);
    g<<fixed<<setprecision(6)<<(sum/prob);
}