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

ok,ng=-1,10**9+1
while abs(ok-ng)>1:
#for _ in range(10):
  mid=(ok+ng)//2
  A=D[0]
  for i in range(1,N):
    B=D[i]
    b=-1
    AA=[]
    for a in A:
      while b+1<M and a+mid>=B[b+1]:
        if a<=B[b+1]:
          AA.append(B[b+1])
        b+=1
    A=sorted(list(set(AA)))
  if len(A)==0:
    ok=mid
  else:
    ng=mid
  #print(ok,ng)
if ng==10**9+1:
  print(-1)
else:
  print(ng)