use std::io::*;
use std::str::FromStr;

#[allow(dead_code)]
fn get_line() -> String {
    let stdin = stdin();
    let mut line = String::new();
    stdin.lock().read_line(&mut line).expect("io error.");
    line.trim().to_string()
}

#[allow(dead_code)]
fn cast<T: FromStr>(s: &str) -> T {
    s.parse().ok().expect("parse error.")
}

#[allow(dead_code)]
fn get_vec<T: FromStr>() -> Vec<T> {
    (&get_line()).split(' ').map(cast::<T>).collect()
}

fn main() {
    let n: u64 = cast(&get_line());

    let v: Vec<i64> = get_vec();
    let ans = v[1] - v[0];

    if ans < 0 {
        println!("-1");
        return;
    }

    for _ in 0..n-1 {
        let v: Vec<i64> = get_vec();
        if v[1] - v[0] != ans {
            println!("-1");
            return;
        }
    }

    println!("{}", ans);
    return;
}