#include <fstream>

using namespace std;

ifstream cin("maxsecv.in");
ofstream cout("maxsecv.out");

int a[1000010],i,j,k,maxx,s,n;

int main()
{
    cin >> n;
    j = 1;
    for (i = 1; i <= n; ++i) a[i] = 0;
    for (i = 1; i <= n; ++i)
    {
     	cin >> k;
        if (k==0 && !(j==1 && a[j]==0)) j++;
        else
        if (k==1) a[j]++;
    }

    maxx = 0;
    for (i = 1; i <= j; ++i)
    	if (maxx < a[i])
        {
        	maxx = a[i];
            k = i;
        }
    a[k] = 0;
    s = maxx;
    maxx = 0;
    for (i = 1; i <= j; ++i)
    	if (maxx < a[i]) maxx = a[i];
    cout << s + maxx;
    return 0;
}