import itertools as iter
import collections as coll
import heapq as hq
import bisect as bis
from decimal import Decimal as dec
from functools import cmp_to_key
import math
import sys
#import pypyjit
#pypyjit.set_param('max_unroll_recursion=-1')
sys.setrecursionlimit(10 ** 6)
inp = sys.stdin.readline
input = lambda : inp().rstrip()
getN = lambda : int(inp())
getNs = lambda : map(int, inp().split())
getList = lambda :list(map(int, inp().split()))
getStrs = lambda n : [input() for _ in [0] * n]
def yexit(): print("Yes"); exit(0)
def nexit(): print("No"); exit(0)
pi = 3.141592653589793
mod = 1000000007
MOD = 998244353
INF = 4611686018427387903
dx = [1, 0, -1, 0];	dy = [0, 1, 0, -1]
#di = coll.defaultdict(int)


"""
Main Code
"""

n, m = getNs()
D = [sorted(getNs()) for _ in [0]*n]

def check(mid):
	stk = [*range(m)]
	visited = set(stk)
	while(stk):
		# print(mid, stk)
		v = stk.pop()
		i = v // m;		j = v % m
		if(i == n - 1):
			return True
		ni = i + 1
		for nj in range(m):
			next = ni * m + nj
			if(next in visited or not(0 <= D[ni][nj] - D[i][j] <= mid)):
				continue
			visited.add(next)
			stk.append(next)
	return False

ok = 10 ** 10
ng = -1
while(ok - ng > 1):
	mid = (ok + ng) // 2
	if(check(mid)):
		ok = mid
	else:
		ng = mid

if(ok > 10 ** 9):
	print(-1)
else:
	print(ok)