n,m=map(int,input().split())
a=[]
for i in range(n):
	a+=[list(map(int,input().split()))]
	a[-1].sort()
from bisect import bisect_left,bisect_right
X=10**9+1
ok=X
ng=-1
while ok-ng>1:
	mid=(ok+ng)//2
	q=[1]*(m+1)
	q[m]=0
	for i in range(1,n):
		nq=[0]*(m+1)
		for j in range(m):
			if q[j]:
				l=bisect_left(a[i],a[i-1][j])
				r=bisect_right(a[i],a[i-1][j]+mid)
				nq[l]+=1
				nq[r]-=1
		for j in range(m):
			nq[j+1]+=nq[j]
			nq[j]=int(nq[j]>0)
		q=nq
	if any(q):
		ok=mid
	else:
		ng=mid
print(ok if ok<X else -1)