// Author: Ivan Kazmenko (gassa@mail.ru)
module solution;
// version = IO_FILES;
import std.algorithm;
import std.range;
import std.stdio;
import std.typecons;

immutable string PROBLEM_NAME = "lights";

alias Lamp = Tuple !(int, q{x}, int, q{r});

bool check (Lamp [] lamps)
{
	int limit = lamps.front.x;
	foreach (ref lamp; lamps)
	{
		if (lamp.x > limit)
		{
			return false;
		}
		limit = max (limit, lamp.x + lamp.r);
	}
	return true;
}

void main ()
{
	version (IO_FILES)
	{
		stdin  = File (PROBLEM_NAME ~ ".in",  "rt");
		stdout = File (PROBLEM_NAME ~ ".out", "wt");
	}

	int n;
	while (readf (" %s", &n) > 0)
	{
		auto lamps = new Lamp [n];
		foreach (ref lamp; lamps)
		{
			readf (" %s %s", &lamp.x, &lamp.r);
		}
		auto lampsRev = lamps.retro
		    .map !(lamp => Lamp (-lamp.x, lamp.r)).array;
		writeln (check (lamps) && check (lampsRev) ?
		    (lamps.back.x - lamps.front.x) * 3L : -1);
	}
}