use proconio::input;

fn main() {
	input! {
		n: usize,
		xy: [(usize, usize); n],
	}
	
    let diffs: Vec<_> = xy.into_iter()
        .map(|(x, y)| y as isize - x as isize)
        .collect();

    let ans: isize = diffs.first().copied()
        .filter(|&first| first >= 0)
        .filter(|&first| diffs.iter().all(|&d| d == first))
        .unwrap_or(-1);

	println!("{ans}");
}