def General_Binary_Increase_Search_Integer(L, R, cond, default=None):
    """ 条件式が単調増加であるとき, 整数上で二部探索を行う.

    L: 解の下限
    R: 解の上限
    cond: 条件(1変数関数, 広義単調増加を満たす)
    default: Lで条件を満たさないときの返り値
    """

    if not(cond(R)):
        return default

    if cond(L):
        return L

    R+=1
    while R-L>1:
        C=L+(R-L)//2
        if cond(C):
            R=C
        else:
            L=C
    return R
#==================================================
def solve():
    N,M=map(int,input().split())

    D=[]
    for i in range(N):
        Di=list(map(int,input().split()))
        Di.sort()
        D.append(Di)

    DP=[[0]*M for _ in range(N)]
    DP[0]=[1]*M

    def check(X):
        for i in range(1,N):
            Di=D[i]; Dii=D[i-1]
            DPi=DP[i]; DPii=DP[i-1]

            l=0; r=0; k=0
            for j in range(M):
                while r<M and Dii[r]<=Di[j]:
                    k+=DPii[r]
                    r+=1
                while l<r and Di[j]-Dii[l]>X:
                    k-=DPii[l]
                    l+=1
                DPi[j]=1 if k else 0

        return any(DP[-1])

    if not check(float("inf")):
        return -1

    return General_Binary_Increase_Search_Integer(0,10**9,check)

#==================================================
print(solve())