#include<fstream>
#include<iomanip>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n,k,w[32],b;
double v[32],a,c;
void bk(int p)
{
    int i,j;
    for (i=w[p-1]+1;i<=p+n-k;i++)
    {
        w[p]=i;
        if(p==k)
        {
            c=1;
            for(j=1;j<=k;j++)
                c*=v[w[j]];
            a+=c;
            b++;
        }
        else
        if(p+1<=k)
            bk(p+1);
    }
}

int main()
{
    int i;
    double sol;
    f>>n>>k;
    for(i=1;i<=n;i++)
        f>>v[i];
    bk(1);
    sol=(double)a/b;
    g<<fixed<<setprecision(6)<<sol;
    return 0;
}