#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

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

double st[35],sum=0;
int i,j,n,m,k,v[35],prob;
inline void bktr(int p,double r,int val)
{
    if(p==k)
    {
        sum+=r;
        prob++;
        return;
    }
    int m=n-(k-p)+1;
    for(int x=val+1;x<=m;x++)
    {
        bktr(p+1,r*st[x],x);
    }
}
int main()
{
    f>>n>>k;
    for(i=1;i<=n;i++){
        f>>st[i];
    }
    bktr(0,1.0,0);
    g<<fixed<<setprecision(6)<<(sum/prob);
}