#include <bits/stdc++.h>

using namespace std;

#define iter(v) v.begin(), v.end()
#define SZ(v) (int)v.size()
#define pb emplace_back
#define ff first
#define ss second

using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

#ifdef zisk
void debug(){cout << endl;}
template<class T, class ... U>
void debug(T a, U ... b){cout << a << " ", debug(b...);}
template<class T> void pary(T l, T r){
	while(l != r) cout << *l << " ", l++;
	cout << endl;	
}
#else
#define debug(...) void()
#define pary(...) void()
#endif

template<class A, class B>
ostream& operator<<(ostream& o, pair<A, B> p){
	return o << '(' << p.ff << ',' << p.ss << ')';
}

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(0);

	int n;
	cin >> n;
	vector<int> dis(n);
	vector<int> e(n);
	for (int i = 1;i < n;i++) {
		cin >> e[i];
		if (e[i] == 1) {
			dis[i] = 1;
		} else {
			dis[i] = dis[i-1] + 1;
		}
	}
	for (int i = n-1;i >= 0;i--) {
		if (e[i] == 1) {
			for (int j = i;j >= 0;j--) {
				dis[j] = min(dis[j], 2);
			}
			break;
		}
	}
	cout << *max_element(dis.begin(), dis.end()) << "\n";
}