from bisect import bisect_left,bisect_right 
from itertools import accumulate
from sys import stdin

def main():
    def is_ok(v):
        S = list(range(len(D[0])))
        for i in range(N-1):
            ND = D[i+1]
            IMOS = [0]*(len(ND)+1)
            for s in S:
                d = D[i][s]
                l = bisect_left(ND,d)
                r = bisect_right(ND,d+v)
                IMOS[l]+=1
                IMOS[r]-=1
            NS = []
            L = list(accumulate(IMOS))
            for j,l in enumerate(L):
                if l>0 and j<len(ND):
                    NS.append(j)
            S = NS
        return S

    N,M = map(int, stdin.readline().split())
    L = [list(map(int, stdin.readline().split())) for _ in range(N)]
    D = []
    for l in L:
        D.append(sorted(list(set(l))))
    ng = -1
    ok = 10**9+100
    while (abs(ok - ng) > 1):
        mid = (ok + ng) // 2
        if is_ok(mid):
            ok = mid
        else:
            ng = mid
    if ok == 10**9+100:
        print(-1)
    else:
        print(ok)

if __name__ == '__main__':
    main()