fn main() {
    let mut input = String::new();
    std::io::Read::read_to_string(&mut std::io::stdin(), &mut input).ok();
    let input: Vec<i64> = input
        .split_whitespace()
        .skip(1)
        .map(|n| n.parse().unwrap())
        .collect();

    let mut n = input[1] - input[0];

    if n <= 0 {
        n = -1;
    } else {
        for xy in input[2..].chunks(2) {
            if xy[1] - xy[0] != n {
                n = -1;
                break;
            }
        }
    }

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