#include <fstream>

using namespace std;
ifstream fi("dezastru.in");
ofstream fo("dezastru.out");
int n,k,A[26],F[26],nr;
double P[26],rez;
void bk(int poz,int last,double sum)
{
    int i;
    if(poz==k+1)
    {
        rez+=sum;
        nr++;
    }
    else
        for(i=last+1;i<=n;i++)
            bk(poz+1,i,sum*P[i]);
}
void citeste()
{
    int i;
    fi>>n>>k;
    for(i=1;i<=n;i++)
        fi>>P[i];
    fi.close();
}
void scrie()
{
    fo<<(double)rez/nr;
    fo.close();
}
int main()
{
    citeste();
    bk(1,0,1);
    scrie();
    return 0;
}