use std::collections::BTreeSet;


fn main() {
    let mut nm = String::new();
    std::io::stdin().read_line(&mut nm).ok();
    let nm: Vec<usize> = nm.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let n = nm[0];
    let m = nm[1];
    let d = (0..n).map(|_| {
            let mut temp = String::new();
            std::io::stdin().read_line(&mut temp).ok();
            temp.trim().split_whitespace().map(|s| s.parse::<isize>().unwrap()).collect::<BTreeSet<_>>()
        })
        .collect::<Vec<_>>();

    let mut lower = 0isize;
    let mut upper = 2000000000 as isize;
    let mut flg = true;
    while upper > lower {
        let middle = (upper + lower) / 2;
        let mut cur = d[0].clone().to_owned();
        for i in 1..n {
            let mut next = BTreeSet::new();
            for &u in cur.iter() {
                d[i].range(u..=u+middle).for_each(|&v| { next.insert(v); });
            }
            cur = next;
            if cur.is_empty() { break; }
        }
        if flg && cur.is_empty() {
            println!("-1");
            return;
        }
        flg = false;
        if cur.is_empty() {
            lower = middle + 1;
        } else {
            upper = middle;
        }
    }
    println!("{}", upper);
}