use std::collections::BTreeSet;

fn lower_bound(target: usize, vals: &Vec<usize>) -> usize {
    let mut upper = vals.len();
    let mut lower = 0usize;
    while upper > lower {
        let middle = (upper + lower) / 2;
        if vals[middle] < target {
            lower = middle+1;
        } else {
            upper = middle;
        }
    }
    upper
}

fn upper_bound(target: usize, vals: &Vec<usize>) -> usize {
    let mut upper = vals.len();
    let mut lower = 0usize;
    while upper > lower {
        let middle = (upper + lower) / 2;
        if vals[middle] <= target {
            lower = middle+1;
        } else {
            upper = middle;
        }
    }
    upper
}

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::<usize>().unwrap()).collect::<BTreeSet<_>>().into_iter().collect::<Vec<_>>()
        })
        .collect::<Vec<_>>();

    let mut lower = 0usize;
    let mut upper = 2_000_000_000usize;
    let mut flg = true;
    while upper > lower {
        let middle = (upper + lower) / 2;
        let mut dp = d[0].clone().to_owned();
        for i in 1..n {
            let mut next_dp = vec![0isize; d[i].len()+1];
            for &v in dp.iter() {
                let left = v;
                let right = v + middle;
                let lidx = lower_bound(left, &d[i]);
                let ridx = upper_bound(right, &d[i]);
                next_dp[lidx] += 1;
                next_dp[ridx] -= 1;
            }
            dp = vec![];
            for j in 0..d[i].len() {
                next_dp[j+1] += next_dp[j];
                if next_dp[j] > 0 {
                    dp.push(d[i][j])
                }
            }
        }
        if flg && dp.is_empty() {
            println!("-1");
            return;
        }
        if dp.is_empty() {
            lower = middle + 1;
        } else {
            upper = middle;
        }
        flg = false;
    }
    println!("{}", upper);
}