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

fn main() {
    let stdin = io::stdin();
    let mut buf = String::new();
    stdin.read_line(&mut buf).ok();

    let mut it = buf.split_whitespace().map(|n| i64::from_str(n).unwrap());
    let num = it.next().unwrap();

    let mut ans = -1000;
    for _ in 0..num {
        let mut buf = String::new();
        stdin.read_line(&mut buf).ok();

        let mut it = buf.split_whitespace().map(|n| i64::from_str(n).unwrap());
        let (a, b) = (it.next().unwrap(), it.next().unwrap());

        let tmp = b - a;
        if ans == -1000 {
            ans = tmp;
            continue;
        }
        if ans != tmp {
            println!("-1");
            return;
        }
    }
    if ans <= 0 {
        println!("{}", -1);
        return;
    }

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