#include <iostream>
#include <fstream>
using namespace std;
int n,k,total;
double d,dez[26];
void rez(int acum,int panaacum,double produs)
{
    int i;
    if (acum==k)
    {
        d+=produs;
        total++;
    }
    else
    {
        for (i=panaacum+1;k-acum<=n-i+1;i++)
        {
            rez(acum+1,i,produs*dez[i]);
        }
    }
}
int main()
{
    ifstream in("dezastru.in");
    ofstream out("dezastru.out");
    int i,j;
    in>>n;
    in>>k;
    for (i=1;i<=n;i++)
    {
        in>>dez[i];
    }
    rez(0,0,1);
    out<<d/total;
    return 0;
}