N,a=open(0)
N=int(N)
a=list(map(int,a.split()))
A=1
for i in range(N-1):
    d=a[i+1]-a[i]
    l=1
    for j in range(N+~i):
        l+=a[j+1]-a[j]==d
    A=max(l,A)
print(A)