#https://aotamasaki.hatenablog.com/entry/meguru_bisect
import bisect
def is_ok(arg):
    now = D[0]
    for i in range(1,N):
        next = []
        for j in range(M):
            ind = bisect.bisect(now,D[i][j])
            if ind != 0:
                if D[i][j]-now[ind-1] <= arg:
                    next.append(D[i][j])
        now = next
        if len(now) == 0:
            return False
    return True


def meguru_bisect(ng, ok):
    '''
    初期値のng,okを受け取り,is_okを満たす最小(最大)のokを返す
    まずis_okを定義すべし
    ng ok は  とり得る最小の値-1 とり得る最大の値+1
    最大最小が逆の場合はよしなにひっくり返す
    '''
    while (abs(ok - ng) > 1):
        mid = (ok + ng) // 2
        if is_ok(mid):
            ok = mid
        else:
            ng = mid
    return ok

N,M = map(int,input().split())
D = [list(sorted(list(map(int,input().split())))) for i in range(N)]
x = meguru_bisect(-1,10**9+1)
if x == 10**9+1:
    print(-1)
else:
    print(x)