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 (pivot < 0 or d[i][pivot] <= d[i+1][j]):
			pivot += 1
		pivot -= 1
		e[i][j] = pivot

dp = [0] * m
for i in range(n-1):
	ndp = [10**18] * (m)
	for j in range(m):
		if e[i][j] >= 0:
			ndp[j] = max(dp[e[i][j]], d[i+1][j] - d[i][e[i][j]])
	dp = ndp

#print(dp)

if min(dp) >= 10**14:
	print(-1)
else:
	print(min(dp))