N = int(input())
A = list(map(int, input().split()))
ans = 1
for i in range(N - 1):
  d = A[i + 1] - A[i]
  for j in range(i + 1, N):
    if A[j] - A[j - 1] != d:
      break
    if ans < j - i + 1:
      ans = j - i + 1
print(ans)