INF = 1 << 60
N, M = map(int, input().split())
D = []
for _ in range(N):
    D.append(sorted(map(int, input().split())))


# 距離 d 以下にできるか
def can(m):
    xs = [True] * M
    for i in range(1, N):
        ys = [False] * M
        p = 0
        for j in range(M):
            if xs[j] == 0: continue
            a = D[i-1][j]

            while p < M and D[i][p] < a:
                p += 1
            while p < M and a <= D[i][p] <= a+m:
                ys[p] = True
                p += 1

        xs = ys

    return any(xs)


lo = 0
hi = INF
ans = hi
while lo <= hi:
    m = (lo + hi) // 2
    if can(m):
        ans = min(ans, m)
        hi = m - 1
    else:
        lo = m + 1

if ans == INF:
    print(-1)
else:
    print(ans)