from typing import List, Tuple, Callable, TypeVar, Optional
import sys
import itertools
import heapq
import bisect
import math
from collections import deque, defaultdict
from functools import lru_cache, cmp_to_key

input = sys.stdin.readline

if __file__ != 'prog.py':
    sys.setrecursionlimit(10 ** 6)


def readints(): return map(int, input().split())
def readlist(): return list(readints())
def readstr(): return input()[:-1]


N, M = readints()
D = [readlist() for _ in range(N)]
for i in range(N):
    D[i].sort()


def judge(A, i, k):
    NA = []
    for d in D[i]:
        l = -1
        r = len(A)
        while r - l > 1:
            c = (l + r) >> 1
            if A[c] <= d:
                l = c
            else:
                r = c
        if l < 0:
            continue
        if d - A[l] <= k:
            NA.append(d)
    return NA


l = -1
r = 10 ** 9 + 10
while r - l > 1:
    c = (l + r) >> 1
    A = D[0]
    for i in range(1, N):
        A = judge(A, i, c)
        if not A:
            l = c
            break
    else:
        r = c
print(r if r < 10 ** 9 + 10 else -1)