#include <iostream.h>
#include <fstream.h>
int main()
{
ifstream f("oo.in");
ofstream g("oo.out");
int i,v[100000],n,s=0,max,p;
f>>n;
for (i=1;i<=n;i++)
    f>>v[i];
f.close();
i=1;
do
  {
  max=0;
  if (v[i]>v[i+1]) {max=v[i];p=i;}
     else {max=v[i+1];p=i+1;}
  
  s+=max+v[p+1];
  v[p+2]=0;
  v[p-1]=0;
  i+=4;
  }
while (i<=n);
g<<s;
g.close();
return 0;
}