#!/usr/bin/evn perl

use strict;
use warnings;

my $n = <>;
my $i = 0;
my @array;

while($i < $n) {
	my($x, $y) = split / /, <>;
	$array[$i] = $y - $x;
	$i++;
}
$i = 1;
while($i < $n) {
	if(($array[$i - 1] != $array[$i])or($array[$i] <= 0)) {
		print "-1\n";
		exit;
	}
	$i++;
}

print "$array[0]\n";

exit;