import sys
sys.setrecursionlimit(5*10**5)
input = sys.stdin.readline
import pypyjit
pypyjit.set_param('max_unroll_recursion=-1')
from collections import defaultdict, deque, Counter
from heapq import heappop, heappush
from bisect import bisect_left, bisect_right
from math import gcd

n,m = map(int,input().split())
d  = [[-1] + list(map(int,input().split())) for i in range(n)]
for i in range(n):
  d[i].sort()
  
m+= 1
_ = [i for i in range(m)]
l = -1
r = 10**9+10
while r-l > 1:
  mid = (r+l)//2
  dp = _[::1]
  for i in range(1, n):
    dd = d[i]
    new = [0]*(m)
    ll = 0
    rr = 0
    for j in range(1,m):
      bot =max(1, d[i][j] - mid)
      top = d[i][j]
      while ll < m-1 and d[i-1][ll+1] < bot:
        ll += 1
      while rr < m-1 and d[i-1][rr+1] <= top:
        rr += 1
        if rr == m: break
      tmp = dp[rr] - dp[ll]
      new[j] = new[j-1] + tmp
    dp = new
  if dp[-1] == 0 :
    l = mid
  else:
    r = mid
ans = r
print(ans) if ans != 10**9+10 else print(-1)