use std::io::Read;

fn main() {
	let mut s = String::new();
	std::io::stdin().read_to_string(&mut s).ok();
	let v: Vec<i64> =
		s.split_whitespace().skip(1).flat_map(str::parse).collect();
	let mut m = 0;
	let r: Vec<i64> = v
		.chunks(2)
		.map(|n| {
			let l = n[0] + 4 * n[1];
			m = m.max(l);
			l
		})
		.collect();
	let mut t = 0;
	for r in r {
		if (m - r) % 2 != 0 {
			t = -1;
			break;
		}
		t += (m - r) / 2;
	}
	println!("{}", t)
}