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