#include <fstream>
#include <cmath>

using namespace std;

ifstream f("oo.in");
ofstream g("oo.out");

int main()
{
    long int n,s=0;
    f>>n;
    short int v[n+2];
    for(int i=1;i<=n;i++)
        f>>v[i];
    v[n+1]=0;v[n+2]=0;
    int j=0;
    bool k=1;
    while(k)
    {
        if(n-j>=4)
        {
            s+=v[j+1]+v[j+2];
            v[j]=0;v[j+1]=0;v[j+2]=0;v[j+3]=0;
            j+=4;
        }
        else
        {
            s+=fmax(v[j]+v[j+1],v[j+1]+v[j+2]);
            j+=4;
        }
        if(j>n)
            k=0;

    }
    g<<s;
    return 0;
}