fn main() {
    let mut xx = String::new();
    std::io::Read::read_to_string(&mut std::io::stdin(), &mut xx).ok();
    let xx: Vec<u64> = xx.split_whitespace().skip(1).flat_map(str::parse).collect();

    let len: Vec<u64> = xx.chunks(2).map(|ab| ab[0] + 4 * ab[1]).collect();

    if len.iter().any(|l| l % 2 != len[0] % 2) {
        println!("-1");
    } else {
        let max_len = len.iter().max().unwrap();
        let answer: u64 = len.iter().map(|l| (max_len - l) / 2).sum();

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