fn main() {
    let stdin = std::io::read_to_string(std::io::stdin()).unwrap();
    let mut stdin = stdin.split_ascii_whitespace();

    let n: usize = stdin.next().unwrap().parse().unwrap();
    let equations: Vec<(i32, i32)> = (0..n)
        .map(|_| {
            (
                stdin.next().unwrap().parse().unwrap(),
                stdin.next().unwrap().parse().unwrap(),
            )
        })
        .collect();

    println!("{}", output(solve(equations)));
}

fn solve(equations: Vec<(i32, i32)>) -> i32 {
    let candidate = equations[0].1 - equations[0].0;
    if candidate.signum() != 1 {
        return -1;
    }

    for (x, y) in equations {
        if candidate + x != y {
            return -1;
        }
    }
    candidate
}

fn output(ans: i32) -> i32 {
    ans
}