#include <bits/stdc++.h>

using namespace std;
ifstream fin("oo.in");
ofstream fout("oo.out");
int n,v[100005];
void citire(){
    fin>>n;
    for(int i=1;i<=n;++i)
        fin>>v[i];
}
int posibil[100005];
int suma1[100005];
int main()
{
    citire();
    int maxim=0;
    int suma=0;
    suma+=v[1]+v[2];
    for(int i=4;i<=n-1;i+=3){
        maxim=0;
        if(posibil[i]!=-1){
        int maxim=max((v[i]+v[i+1]),max(v[i+1]+v[i+2],v[i+2]+v[i+3]));
        suma+=maxim;
        if(maxim==(v[i]+v[i+1]))posibil[i+2]=posibil[i]=posibil[i+1]=-1;
        else if(maxim==v[i+1]+v[i+2])posibil[i+3]=posibil[i+1]=posibil[i+2]=-1;
        else if(maxim==v[i+2]+v[i+3])posibil[i+2]=posibil[i+3]=posibil[i+4]=-1;
        }
    }
    fout<<suma;
    return 0;
}