#include <fstream>
#include <iomanip>
#define VAL 35

using namespace std;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

int N, K, S, i;
int biti[VAL];
double v[VAL], sum;
double fact, P, a, b;

int main()
{
    fin >> N >> K;
    a=b=1;
    for (i=1; i<=N; i++)
    {
        fin >> v[i];
        a*=i;
        if (i<=K)
          b*=i;
    }
    P=1;
    while (S!=N)
    {
        i=1;
        while (biti[i]==1)
        {
            biti[i]=0;
            S--;
            P/=v[i];
            i++;
        }
        biti[i]=1;
        P*=v[i];
        S++;
        if (S==K)
          sum+=P;
    }
    sum*=b;
    sum/=a;
    fout << fixed << setprecision(6) << sum << '\n';
    fin.close();
    fout.close();
    return 0;
}