import bisect
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()
# print(*d, sep = "\n")
lim = 1 << 30
l, r = -1, lim
while r - l > 1:
  piv = (l + r) // 2
  dp = [[0] * m for i in range(n)]
  dp[0] = [1] * m
  for i in range(n-1):
    idx = 0
    for j in range(m):
      while idx < m:
        if d[i][idx] > d[i+1][j]:
          idx -= 1
          break
        idx += 1
      if idx == -1:
        idx = 0
        continue
      idx = min(idx, m-1)
      if d[i+1][j] - d[i][idx] <= piv:
        dp[i+1][j] |= dp[i][idx]
  # print(l, r, piv)
  # print(*dp, sep = "\n")
  if sum(dp[-1]):
    r = piv
  else:
    l = piv
print(r if r < lim else -1)