n,m=map(int,input().split())
s=[list(map(int,input().split()))for i in range(n)]
for i in range(n):s[i].sort()
from bisect import*
def le(a, x):
    i = bisect_right(a, x)
    if i:return a[i-1]
    return -1
def ge(a, x):
    i = bisect_left(a, x)
    if i != len(a):return a[i]
    return -1
def validate(x):
    ans=10**18
    for i in range(m):
        c=s[0][i]
        k=0
        for j in range(1,n):
            r=le(s[j],c+x)
            if r==-1 or r<c:break
            k=max(k,r-c)
            c=r
        else:ans=min(ans,k)
    if ans==10**18:return True
    else:return False
def binarysearch(left=-1,right=10**18,r=0):
    while left<=right:
        mid=(left+right)//2
        if validate(mid):r=mid;left=mid+1
        else:right=mid-1
    return r
c=binarysearch()
if c==10**18:
    ans=10**18
    for i in range(m):
        c=s[0][i]
        k=0
        for j in range(1,n):
            r=ge(s[j],c)
            if r==-1:break
            k=max(k,r-c)
            c=r
        else:ans=min(ans,k)
    if ans==10**18:print(-1)
    else:print(ans)
else:print(c+1)