from bisect import bisect_left
import sys
input = sys.stdin.readline

n, m = map(int, input().split())
D = []
for _ in range(n):
    L = list(map(int, input().split()))
    L.sort()
    D.append(L)

INF = 10**18
DP = [[INF for _ in range(m)] for _ in range(n)]
for i in range(m):
    DP[0][i] = 0
for i in range(n - 1):
    for j in range(m):
        if DP[i][j] == INF:
            continue
        prev_d = D[i][j]
        idx = bisect_left(D[i + 1], prev_d)
        if idx == m:
            continue
        res = max(DP[i][j], D[i + 1][idx] - prev_d)
        DP[i + 1][idx] = min(DP[i + 1][idx], res)
ans = min(DP[n - 1])
if ans == INF:
    ans = -1
print(ans)