<?php
$count= trim(fgets(STDIN));
$number = array();
$answer = 0;
for ($i = 0; $i < $count; $i++) {
     $number = explode(" ", trim(fgets(STDIN)));
     $answer = $number[1] - $number[0];
     $array[] = $answer;
}
// print_r($array);
foreach ($array as $key => $value){
    if ($array[$key] == $array[$key - 1]) {
        $ans = $array[0];
    } else {
        $ans = "-1";
    }
}
echo $ans;