#include <fstream>
#include <iomanip>

using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");

double P[26],sum;
int N,KK,x[26],nr;

void BT(int k,double prob)
{
    if (k<=KK)
    {
        for (int i=x[k-1]+1;i<=N-KK+k;++i)
        {
            x[k]=i;
            BT(k+1,prob*P[i]);
        }
    }
    else
    {
        sum+=prob;
        ++nr;
    }
}

int main()
{
    f>>N>>KK;
    for(int i=1;i<=N;++i)
        f>>P[i];
    BT(1,1.0);
    g<<fixed<<setprecision(6)<<sum/nr;
    f.close();
    g.close();
    return 0;
}