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