#include <bits/stdc++.h>
using namespace std;
const int N = 1000005;
int n;
int token[N];
int far[N], ans[N];

int main() {
	scanf("%d", &n);
	for (int i = 2; i <= n; ++i) scanf("%d", &token[i]);
	far[2] = ans[2] = 1;
	for (int i = 3; i <= n; ++i) {
		if (token[i]) {
			ans[i] = max(min(ans[i - 1], 2), 1);
			far[i] = 1;
		} else {
			ans[i] = max(ans[i - 1], far[i - 1] + 1);
			far[i] = far[i - 1] + 1;
		}
	}
	printf("%d\n", ans[n]);
	return 0;
}