#include <fstream>

#include <iomanip>

using namespace std;

ifstream cin("dezastru.in");

ofstream cout("dezastru.out");

int n,k;

double pb[30],newp,sol,nr;

int v[30];

void construct(int p,double prob)

{

    if(p==k+1)

    {

        sol+=prob;

        nr+=1.0;

        return ;

    }

    for(int i=v[p-1]+1;i<=n;++i)

    {

        newp=prob*pb[i];

        v[p]=i;

        construct(p+1,newp);

    }

}

int main()

{

    cin>>n>>k;

    for(int i=1;i<=n;++i)

        cin>>pb[i];

    construct(1,1.0);

    cout<<fixed<<setprecision(7)<<sol/nr;

    return 0;

}