#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int N,K;
double Probability=1;
int Fact=1,first;
double Array[30];
int x[30],counter=1,counter2;
double sum;
double aux=1;
void Factorial()
{
    int i=1;
    for(i=1;i<=N;i++)
        Fact*=i;
}
void Read()
{
    int i;
    f>>N>>K;
    for(i=1;i<=N;i++)
        f>>Array[i];
}
void Number()
{
    int i;
    for(i=N;i>=N-K+1;i--)
        counter*=i;
}
void back(int k)
{
    first=k;
    int i;
    for(i=x[k-1]+1;i<=N-K+k;i++)
    {
        if(i==x[k-1]+1)
            x[k]=i;
        else
        {
            x[k]=i;
            aux/=Array[i-1];
        }
        aux*=Array[x[k]];
        if(k==K)
        {
            sum+=aux;
            counter2++;
        }

        else
        {
            back(k+1);
        }
    }
    aux/=Array[N-K+k];
}
int main()
{
    Read();
    Factorial();
    Number();
    back(1);
    Probability=(sum*(counter/counter2))/(double)counter;
    g<<fixed<<setprecision(6)<<Probability<<"\n";
    return 0;
}