<?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);
$OK = 0;
$NG = 0;
foreach ($array as $key => $value){
    if ($array[$key] == $array[$key+1]) {
        $OK ++;
    } else {
        $NG ++;
    }
}
if ($OK == $count-1) {
    echo $array[0];
} else {
    echo "-1";
}