fn main() {
    let mut n = String::new();
    std::io::stdin().read_line(&mut n).ok();
    let n: usize = n.trim().parse().unwrap();
    let mut lines: Vec<usize> = vec![];
    for _ in 0..n {
        let mut st = String::new();
        std::io::stdin().read_line(&mut st).ok();
        let st: Vec<usize> = st.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
        lines.push(st[0] + 4 * st[1]);
    }
    let ok: bool = lines.iter().map(|i| i % 2).sum::<usize>() % n == 0;

    if ok {
        let max_val: usize = *lines.iter().max().unwrap();
        println!("{}", lines.iter().map(|i| max_val - i).map(|i| i / 2).sum::<usize>())
    } else {
        println!("-1");
    }
}