#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

// TÚL SOKSZOR FUT LE A REKURZÍV FÜGGVÉNY, NEM JÓ

template<typename Iter>
int maxKorong(Iter k, size_t n, int balPont, int jobbPont, int korszam) {
    //for(int i = 0; i < n; i++) {
        //cout << k[i] << " ";
    //}
    //cout << jobbPont << " " << korszam << endl;
        int balPontS = balPont, jobbPontS = jobbPont;
    if(korszam%2==0) {
        if (n == 1) return k[0] == 1 ? max(++balPont, ++jobbPont) : max(balPont, jobbPont);
        if(k[0] == 1) balPontS++;
        if(k[n-1] == 1) jobbPontS++;
    } else if (n==1) return max(balPont, jobbPont);
    int subArrayLeft[n-1]; int subArrayRight[n-1];
    for(int i = 0; i <= n - 1; i++) {
        if(i==0) subArrayLeft[0] = k[0];
        else if(i==n-1) subArrayRight[i-1] = k[i];
        else {
            subArrayLeft[i] = k[i];
            subArrayRight[i-1] = k[i];
        }
    }
    ++korszam;
    return max(maxKorong(k, n-1, balPont, jobbPontS, korszam), maxKorong(k+1, n-1, balPontS, jobbPont, korszam));
}

int main()
{
    int n;
    cin >> n;
    int korongok[n];
    for(int i = 0; i < n; i++) {
        cin >> korongok[i];
    }
    cout << maxKorong(korongok, n, 0, 0, 0);
    return 0;
}