<?php
$n = trim(fgets(STDIN));
$a = array();
for ($i = 0; $i < $n; $i++) {
	list($x, $y) = explode(' ', trim(fgets(STDIN)));
	$a[] = $y - $x;
}
sort($a);
echo $a[0] == $a[$n - 1] ? $a[0] && $a[0] > 0 : '-1';
?>