import sys
#input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

n, m = na()
a = [sorted(na()) for i in range(n)]
if n == 1:
    print(0)
    exit()
ok = 10**9+1
ng = -1
from collections import deque

while ok-ng > 1:
    d = ok+ng>>1
    dp = [1] * m
    for i in range(n-1):
        k = 0
        dq = deque()
        ndp = [0] * m
        dp, ndp = ndp, dp
        for j in range(m):
            while k < m and a[i+1][k] <= a[i][j] + d:
                dq.append(k)
                k += 1

            while dq and a[i+1][dq[0]] < a[i][j]:
                dq.popleft()
            #print(dq)
            if ndp[j]:
                while dq:
                    dp[dq.popleft()] = 1
        #print(dp)
    #print(d, dp)
    if sum(dp):
        ok = d
    else:
        ng = d
if ok == 10**9+1:
    print(-1)
else:
    print(ok)