#include <bits/stdc++.h>

#define _debugVar(x) { cerr << #x << " = " << x << endl; }

using namespace std;
using LL = long long;

const int N = 2e5+5;

int n;
int a[N];
int after[N];

int main()
{
	// freopen("1.in", "r", stdin);
	ios::sync_with_stdio(false);
	cin.tie(nullptr);

	cin >> n;
	for(int i = 2; i <= n; i++) 
		cin >> a[i];

	for(int i = n; i >= 1; i--) {
		after[i] = a[i]+after[i+1];
	}

	int las = 0; // 前面连续的 0
	int ans = 0;
	for(int i = 2; i <= n; i++) {
		if(a[i] == 1) {
			ans = max(ans, 1);
			las = 0;
		} else {
			int res = las+2;
			if(i-las-1 == 1) res = min(res, las+1);
			if(after[i] > 0) res = min(res, 2);
			ans = max(ans, res);
			// _debugVar(las);
			// _debugVar(res);
			las++;
		}
	}

	cout << ans << endl;
	return 0;
}