import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = (1 << 63)-1
# inf = (1 << 31)-1
# md = 10**9+7
md = 998244353

from bisect import bisect

def binary_search(l, r, minimize):
    if minimize: l -= 1
    else: r += 1
    while l+1 < r:
        m = (l+r)//2
        if ok(m) ^ minimize: l = m
        else: r = m
    if minimize: return r
    return l

def ok(m):
    now = dd[0]
    for ds in dd[1:]:
        i = 0
        pre, now = now, []
        for d in ds:
            while i < len(pre) and pre[i] < d-m: i += 1
            if i == len(pre): break
            if pre[i] <= d: now.append(d)
        if not now: return False
    return True

n, m = LI()
dd = LLI(n)
for i in range(n): dd[i].sort()

ans = binary_search(0, 10**9, True)
if ans == 10**9: ans = -1

print(ans)