#include <bits/stdc++.h>
using namespace std;
int arr[111];
int main() {
  int n;
  scanf("%d", &n);
  arr[0] = 0;
  for (int i = (1); i < (n + 1); ++i) scanf("%d", &arr[i]);
  int ans = 0;
  if (n % 2 == 0 || n == 1) {
    printf("-1\n");
    return 0;
  }
  for (int i = n; i >= 1; i -= 2) {
    int tmp = max(arr[i], arr[i - 1]);
    ans += tmp;
    arr[i] = arr[i - 1] = 0;
    arr[(i - 1) / 2] = max(0, arr[(i - 1) / 2] - tmp);
  }
  printf("%d\n", ans);
  return 0;
}
