sub ackermann_c {
    use constant INF =>  exp(~0 >> 1);
    my( $m, $n ) = @_;
    --$m, $n=1         if $n == 0;
    return $n + 1      if $m == 0;
    return $n + 2      if $m == 1;
    return 2*( $n+3)-3 if $m == 2;
    return 2**($n+3)-3 if $m == 3;
    return INF     unless $m == 4;
    my( $p, $r ) = ( $n+3, 2 );
    $r = 2**$r while --$p;
    return $r - 3;
}