#include<iostream>
#include<vector>
using namespace std;

int main(){
  int n;
  while(cin >> n,n){
    int sum = 0;
    for(int i=0; i<n/4; ++i){
      int tmp;
      cin >> tmp;
      sum += tmp;
    }
    cout << sum << endl;
  }
  return 0;
}

