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();
            let temp: Vec<usize> = temp.trim().split_whitespace().map(|s| s.parse::<usize>().unwrap()).collect::<BTreeSet<usize>>().into_iter().collect();
            temp
        })
        .collect::<Vec<_>>();

    let mut upper = 2 * 1e9 as usize;
    let mut lower = 0usize;
    let mut at_first = true;
    while upper > lower {
        let middle = (upper + lower) / 2;
        let mut dp = d[0].clone();
        for i in 1..n {
            let mut next_dp = vec![0isize; d[i].len()+1];
            for &v in dp.iter() {
                let l = lower_bound(v, &d[i]);
                let r = upper_bound(v+middle, &d[i]);
                next_dp[l] += 1;
                next_dp[r] -= 1;
            }
            for i in 0..d[i].len() {
                next_dp[i+1] += next_dp[i];
            }
            dp = vec![];
            for j in 0..d[i].len() {
                if next_dp[j] > 0 {
                    dp.push(d[i][j]);
                }
            }
            if dp.is_empty() {
                break;
            }
        }
        if dp.is_empty() {
            if at_first {
                println!("-1");
                return;
            }
            lower = middle + 1;
        } else {
            upper = middle;
        }
        at_first = false;
    }
    println!("{}", upper);
}