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)

def calc(val):
    DP = D[0].copy()
    for i in range(n - 1):
        NDP = []
        j = 0
        for curr in DP:
            while j < m:
                if D[i + 1][j] < curr:
                    j += 1
                elif D[i + 1][j] > curr + val:
                    break
                else:
                    NDP.append(D[i + 1][j])
                    j += 1
        if len(NDP) == 0:
            return False
        DP = NDP
    return True

l, r = -1, 10**9
if not calc(r):
    print(-1)
    exit()

while r - l > 1:
    mid = (l + r) // 2
    if calc(mid):
        r = mid
    else:
        l = mid
print(r)