#include <fstream>

using namespace std;
ifstream f("maxsecv.in");
ofstream g("maxsecv.out");
long n,v[1000000],s[1000000];
int main()
{
    int i,l=0,ok=0,aux;
    f>>n;
    for(i=1;i<=n;i++) f>>v[i];
        if((v[1]==0)&&(v[n]==0))
           for(i=2;i<=n;i++)
               if(v[i]!=0) ok++;
               else {l++;s[l]=ok;ok=0;}
        else if((v[1]==0)||(v[n]==0))
           for(i=1;i<=n;i++)
               if(v[i]!=0) ok++;
               else {l++;s[l]=ok;ok=0;}
        else
           for(i=1;i<=n+1;i++)
               if(v[i]!=0) ok++;
               else {l++;s[l]=ok;ok=0;}
    for(i=1;i<=l;i++)
        if(s[i]<s[i+1])
    {
        aux=s[i];
        s[i]=s[i+1];
        s[i+1]=aux;
    }
    g<<s[1]+s[2];
    f.close();
    g.close();
    return 0;
}