#include <bits/stdc++.h>
#define int long long

using namespace std;

int n;
int arr[1000005];
int dp[1000005];
int ans;
int mini;

signed main() {
    ios::sync_with_stdio(0), cin.tie(0);
    cin >> n;
    for (int i = 1; i < n; i++) {
        cin >> arr[i];
    }
    fill(dp, dp+n+1, 1e18);
    dp[1] = 0LL;
    dp[0] = 0LL;
    for (int i = 1; i < n; i++) {
        if (arr[i]) {
            if (!dp[i]) dp[i+1] = mini+1;
            else dp[i+1] = max(mini+1, dp[i]);
        }else {
            dp[i+1] = max(mini+1, dp[i]+1);
        }
        if (mini) mini = min(mini, dp[i+1]);
        else mini = dp[i+1];
        ans = max(dp[i+1], ans);
        //cout << i+1 << " " << dp[i+1] << " " << mini << "\n";
    }
    cout << ans;
    return 0;
}