import sys


def recurse(i, l, nodes):
    m = i
    if i in nodes: return 0
    nodes.add(i)
    for j in range(0, len(l)):
        if i != j and l[i] + l[j] == abs(j - i):
            m = max(m, recurse(j, l, nodes))
    return m

while 1:
    n = int(input())
    if n==0: break
    l = [int(i) for i in input().split()]

    # TODO: REAKTOR FAGOTRY
    # TODO: fikslejtr

    print(recurse(0, l, set()))