#!/usr/bin/perl

use strict;
use warnings;

my $n = <>;
my @arraya;
my @arrayb;
my @count;
for(my $i = 0; $i < $n; $i++) {
	($arraya[$i], $arrayb[$i] )= split / /, <>;
	$count[$i] = $arraya[$i] + 4 * $arrayb[$i];
}

@count = reverse sort {$a <=> $b} @count;
my $ans = 0;

for(my $i = 1; $i < $n; $i++) {
	if(($count[0] - $count[$i]) % 2 == 0) {
		my $c = $count[0] - $count[$i];
		$ans += $c / 2;
	} else {
		$ans = -1;
	}
}

if($ans == -1) {
	print "-1\n";
} else {
	print "$ans\n";
}

exit;