using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (n, m) = (c[0], c[1]);
        var d = NArr(n);
        for (var i = 0; i < n; ++i) Array.Sort(d[i]);
        var ok = 1_000_000_000;
        var ng = -1;
        if (!Asakatu(d, ok))
        {
            WriteLine(-1);
            return;
        }
        while (ok - ng > 1)
        {
            var mid = (ok + ng) / 2;
            if (Asakatu(d, mid)) ok = mid;
            else ng = mid;
        }
        WriteLine(ok);
    }
    static bool Asakatu(int[][] d, int cliff)
    {
        var cur = Enumerable.Repeat(1, d[0].Length).ToArray();
        for (var i = 1; i < d.Length; ++i)
        {
            var next = new int[d[i].Length];
            for (var j = 0; j < cur.Length; ++j) if (cur[j] > 0)
            {
                var p = LowerBound(0, d[i - 1][j], d[i]);
                var q = LowerBound(0, d[i - 1][j] + cliff + 1, d[i]);
                if (p < next.Length) ++next[p];
                if (q < next.Length) --next[q];
            }
            for (var j = 1; j < next.Length; ++j) next[j] += next[j - 1];
            cur = next;
        }
        return cur.Any(ci => ci > 0);
    }
    static int LowerBound(int left, int min, int[] list)
    {
        if (list[left] >= min) return left;
        var ng = left;
        var ok = list.Length;
        while (ok - ng > 1)
        {
            var center = (ng + ok) / 2;
            if (list[center] < min) ng = center;
            else ok = center;
        }
        return ok;
    }
}