N = int(input())
A = list(map(int, input().split()))

ans = 1

for left_idx in range(N - 1):
  diff = A[left_idx + 1] - A[left_idx]
  for right_idx in range(left_idx + 1, N):
    if A[right_idx] - A[right_idx - 1] != diff:
      break
    if ans < right_idx - left_idx + 1:
      ans = right_idx - left_idx + 1

print(ans)