n, m = map(int,input().split())
d = [list(map(int,input().split())) for i in range(n)]
for i in range(n):
	d[i].sort()

e = [[0] * m for i in range(n-1)]
for i in range(n-1):
	pivot = 0
	for j in range(m):
		while pivot < m and d[i][pivot] <= d[i+1][j]:
			pivot += 1
		e[i][j] = pivot

suki = 10 ** 9 + 2
kirai = -1
while suki - kirai > 1:
	targ = (suki + kirai) // 2
	dp = [1] * m
	for i in range(n-1):
		pfix = [0] * (m+1)
		for j in range(m):
			pfix[j+1] = pfix[j] + dp[j]
		ndp = [0] * m
		pivot = 0
		f = [0] * m
		for j in range(m):
			while pivot < m and d[i][pivot] < d[i+1][j] - targ:
				pivot += 1
			f[j] = pivot
		for j in range(m):
			v = pfix[e[i][j]] - pfix[f[j]]
			if v >= 1: ndp[j] = 1
		dp = ndp
		#print(*dp)
	if sum(dp) == 0:
		kirai = targ
	else:
		suki = targ

if kirai >= 10 ** 9:
	print(-1)
else:
	print(suki)