n, m = map(int, map(int, input().split()))
d = [list(map(int, input().split())) for _ in range(n)]
for i in range(n):
    d[i].sort()
ok = 10**9
ng = -1
while ok - ng > 1:
    mid = (ok + ng) // 2
    bs = [True] * m
    for i in range(n-1):
        dp = [0] * (m+1)
        k1 = 0
        k2 = 0
        for j in range(m):
            if not bs[j]:
                continue
            while k1 < m and d[i+1][k1] - d[i][j] < 0:
                k1 += 1
            while k2 < m and d[i+1][k2] - d[i][j] <= mid:
                k2 += 1
            dp[k1] += 1
            dp[k2] -= 1
        for j in range(m):
            bs[j] = dp[j] > 0
            dp[j+1] += dp[j]
    if any(bs):
        ok = mid
    else:
        ng = mid
print(-1 if ok == 10**9 else ok)