use proconio::input;

fn main() {
	input! {
		n: usize,
		mut xy: [(i32, i32); n],
	}

	let mut ans: i32 = -2;
	xy.push(xy[0]);
	for (x, y) in xy {
		ans = if ans == -2 {
			y-x
		} else if ans != y-x || x >= y{
			-1_i32
		} else {
			ans
		};
	}

	println!("{}", ans);
}