use std::collections::{BTreeMap, HashSet, BTreeSet};

const INF: usize = 1usize << 60;

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 items = (0..n).map(|_| {
            let mut temp = String::new();
            std::io::stdin().read_line(&mut temp).ok();
            let mut temp = temp.trim().split_whitespace().map(|s| s.parse().unwrap()).collect::<HashSet<usize>>().into_iter().collect::<Vec<_>>();
            temp.sort();
            temp
        })
        .collect::<Vec<_>>();

    let mut lower = 0usize;
    let mut upper = INF;
    let mut flg = true;
    while upper > lower {
        let middle = (upper + lower) / 2;
        let mut dp = BTreeSet::new();
        for &v in items[0].iter() {
            dp.insert(v);
        }
        for i in 1..n {
            let mut next_dp = BTreeSet::new();
            for &v in items[i].iter() {
                let minval = if v < middle { 1 } else { v - middle };
                if let Some(_) = dp.range(minval..=v).nth(0) {
                    next_dp.insert(v);
                }
            }
            if flg && next_dp.is_empty() {
                println!("-1");
                return;
            }
            dp = next_dp;
        }
        flg = false;
        if dp.is_empty() {
            lower = middle + 1;
        } else {
            upper = middle;
        }
    }
    println!("{}", upper);
}