#include <fstream.h>

double a[30],p;
long nr,n,kk;

void bk(int ant, int k, double s){
     int i;
     if (k==kk)p+=s, ++nr;
     else for (i=ant+1;i<n-(kk-k);i++) bk(i,k+1,s*a[i]);
}
     


int main()
{int i,k;
 ifstream f("dezastru.in");
 freopen ("dezastru.out", "w",stdout);
 f>>n>>kk;
 for (i=1;i<=n;i++) f>>a[i];
 bk(1,0,1);
 printf("%lf",p/nr);
 f.close();
}