A=[*map(int,open(0).read().split())];a=1
for l in range(1,A[0]):
  d=A[l+1]-A[l]
  for r in range(l+1, A[0]+1):
    if A[r]-A[r-1]!=d:
      break
    a=max(a,r-l+1)
print(a)