from itertools import groupby

N, *A = map(int, open(0).read().split())

G = [len(tuple(v)) for _, v in groupby(A)]

dp = [0] * (len(G) + 2)
for i, g in enumerate(G):
    dp[i + 1] = max(dp[i + 1], dp[i] + g - 1)
    dp[i + 2] = max(dp[i + 2], dp[i] + g)

print(max(dp))