use proconio::input;

fn main() {
    input! {
        n:usize,
        xy: [(i64, i64);n],
    }
    let (x, y): (Vec<_>, Vec<_>) = xy.into_iter().unzip();

    let answer:i64 = y[0] - x[0];
    let mut equal = true;
    for i in 1..n{
        if answer != y[i] - x[i]{
            equal = false;
        }
    }
    if equal{
        println!("{}",answer)
    }else{
        println!("-1")
    }
}