#include <stdio.h>
#include <iostream>
using namespace std;

int main()
{
    freopen("maxsecv.in","r",stdin);
    freopen("maxsecv.out","w",stdout);

    int n, i, j, x, sec1, sec2, maxim1, maximm2;

    scanf("%d",&n);
    sec1 = 0; sec2 = 0; maxim1 = 0; maximm2 = 0;

    for( i = 1; i <= n; ++i ){
        scanf("%d",&x);
        if( x == 1 ){
            sec1 = 0;
            while( x == 1 && i <= n ){
                scanf("%d",&x);
                sec1++;
                i++;
            }
            if( maximm2 == 0 ) maximm2 = sec1;
            if( sec1 >= maxim1 ){
                maximm2 = maxim1;
                maxim1 = sec1;
            }
            if( sec1 > maximm2 && sec1 < maxim1 ) maximm2 = sec1;

        }
        else sec1 = 1;

    }
    printf("%d",maxim1+maximm2);


    return 0;
}