#! perl -slw
use strict;

sub overlap{
	@_ = map{abs $_ } @_ if 4 == grep $_ < 0, @_;
	local $_ = ($_[1] < $_[3] ? $_[1]: $_[3]) - ($_[0] > $_[2] ? $_[0] : $_[2]) ;
	$_ > -1 ? $_ : 0;
}

while(<DATA>) {
	chomp;
	print,next unless length;
	print $_, ' = ', overlap( ( split(' ') )[0 .. 3] );
}

print'';

my @data = qw[1 5 2 6];

print "@data : ", overlap @data
	and @data = map($_-1, @data)
		while grep $_>0,@data;

__DATA__
1 2 2 3 (0)
2 3 1 2 (0)
1 2 3 4 (0)
1 3 2 4 (1)
1 4 2 3 (1)
2 4 1 3 (1)
3 4 1 2 (0)

-2 -1 1 2 (0)
-2 1 -1 2 (2)
-2 2 -1 1 (2)
-1 2 -2 1 (2)
1 2 -2 -1 (0)

-2 -3 -1 -2 (0)
-1 -2 -3 -4 (0)
-1 -3 -2 -4 (1)
-1 -4 -2 -3 (1)
-2 -4 -1 -3 (1)
-3 -4 -1 -2 (0)