#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
	cin.tie(nullptr)->sync_with_stdio(false);
	int n;
	cin >> n;
	vector<int>dp(n+2);
	vector<int>a(n+2);
	vector<int>dp2(n+1);
	for(int i = 2; i<=n; i++){
		cin >> a[i];
	}
	int ans = 0;
	dp[n+1] = (int)1e9;
	for(int i = n; i>=1; i--){
		dp[i] = dp[i+1]+1;
		if(a[i] == 1){
			dp[i] = 0;
		}
	}
	dp2[0] = (int)1e9;
	for(int i = 1; i<=n; i++){
		dp2[i] = dp2[i-1]+1;
		if(a[i] == 1){
			dp2[i] = 0;
		}
		if(i>1){
			int v = min({i-1,dp2[i]+1,dp[i]+1});
			ans = max(ans,v);
		}
	}
	cout << ans << '\n';
	return 0;
}