<>;
$_=<>;
($a,$b)=/\d+/g;
$x=$a+4*$b;
for(<>){
    ++$l;
    ($a,$b)=/\d+/g;
    $y=$a+4*$b;
    if($x+$y&1){
	print -1;
	exit;
    }
    if($x<$y){
	$s+=$l*($y-$x);
	$x=$y;
    }else{
	$s+=$x-$y;
    }
}
print $s/2;