# step 1
sub binary {
   my ($n) = @_;
   return $n if $n == 0 || $n == 1;
#step 2
   my $k = int($n/2);
   my $b = $n % 2;
#step 3, recursion
   my $E = binary($k);
#step 4
   return $E . $b;
   }