import sys
input = sys.stdin.readline

N, M = map(int, input().split())
D = []
for i in range(N):
    D.append(list(map(int, input().split())))
    D[i].sort()

def check(m):
    pre = [1] * M
    Dpre = D[0]
    for i in range(1, N):
        dp = [0] * M
        Di = D[i]
        now = 0
        for j in range(M):
            while now < M:
                if pre[now] and Dpre[now] <= Di[j] <= Dpre[now] + m:
                    dp[j] = 1
                    break
                now += 1
        dp, pre = pre, dp
        Dpre, Di = Di, Dpre
    return sum(pre)  
    
yes = 10 ** 9 + 5
no = -1
if not check(yes):
    print(-1)
    exit()

while yes - no != 1:
    mid = (yes + no)//2
    if check(mid):
        yes = mid
    else:
        no = mid
print(yes)