#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int stc[10001],f[100],n,k,l;
double a[100],xd[100],xr,sum;
void permutare(int nivel,int u)
{
    if(nivel==k+1)
    {
        xr=1;
        for(int i=1; i<=k; i++)
        {
            //cout<<stc[i]<<" ";
            xr*=a[stc[i]];
        }
        //cout<<xr;
        l++;
        xd[l]=xr;

        //cout<<"\n";
        return;
    }
    for(int i=u+1;i<=n-k+nivel;i++)
    {
        stc[nivel]=i;
        permutare(nivel+1,i);
    }
}
int main()
{
    fin >>n>>k;
    for(int i=1; i<=n; i++)
        fin>>a[i];
    permutare(1,0);
    for(int i=1;i<=l;i++)
    {
        sum+=xd[i];
    }
    fout<<sum/l;
    return 0;
}