#include <fstream>
#include <iomanip>
using namespace std;
ifstream cin("dezastru.in");
ofstream cout("dezastru.out");
int n,k,perm;
double v[26],sol;
void bck(int p,int last,double val){
    if(p==k+1){
        sol+=val;
        perm++;
        return ;
    }
    for(int i=last+1;i<=n;i++)
        bck(p+1,i,val*v[i]);
}
int main()
{
    cin>>n>>k;
    for(int i=1;i<=n;i++)
        cin>>v[i];
    bck(1,0,1.0);
    cout<<fixed<<setprecision(7)<<sol/perm;
    return 0;
}