sub A
 {
 return 0  if $_[1] == 0; 
 return 2*$_[1]  if $_[0] == 0;
 return 2  if $_[1] == 1;
 my $mm= $_[0]-1;
 --$_[1];
 $_[1]= &A;
 $_[0]= $mm;
 return &A;
 }