import bisect
def is_ok(v):
    S = set(range(len(D[0])))
    for i in range(N-1):
        ND = D[i+1]
        NS = set()
        for s in S:
            d = D[i][s]
            l = bisect.bisect_left(ND,d)
            r = bisect.bisect_right(ND,d+v)
            for j in range(l,r):#TLEするようならimos使う
                NS.add(j)
        S = NS
    return S

def meguru_bisect(ng, ok):
    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 = []
for i in range(N):
    D.append(sorted(list(set(map(int, input().split())))))
ans = meguru_bisect(-1, 10**9 + 100)
if ans == 10**9+100:
    print(-1)
else:
    print(ans)