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 ge(a, x):
    i = bisect_left(a, x)
    if i != len(a):return a[i]
    return -1
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)