import sys
#input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

n, m = na()
d = [sorted(na()) for i in range(n)]
if n == 1:
    print(0)
    exit()
from bisect import bisect_left as bl
ans = float("inf")
for i in range(m):
    x = d[0][i]
    z = 0
    for j in range(1, n):
        y = bl(d[j], x)
        if y == m:
            break
        z = max(z, d[j][y]-x)
        x = d[j][y]
    else:
        ans = min(ans, z)

if ans == float("inf"):
    print(-1)
else:
    print(ans)