#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int n,t,ns;
double a[30],sum;
static inline void bk(int k, int b, double p)
{
    if(k==t)
    {
        ++ns;
        sum+=p;
        return;
    }
    for (int i=b+1; i<=n+k-t+1; ++i)
        bk(k+1,i,p*a[i]);
}

int main()
{

    fin>>n>>t;
    for(int i=1;i<=n;i++)
        fin>>a[i];
    bk(0,0,1);
   fout<<(sum/double(ns));
    return 0;
}