#include <bits/stdc++.h>
using namespace std;
template <class T>
inline T bigmod(T p, T e, T M) {
  long long int ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p) % M;
    p = (p * p) % M;
  }
  return (T)ret;
}
template <class T>
inline T modinverse(T a, T M) {
  return bigmod(a, M - 2, M);
}
template <class T>
inline T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
int arr[105], cnt;
int moveee(int n) {
  for (int i = 3, j = 1; i <= 99; i += 2, j++)
    if (n == i) return j;
  return -1;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  ;
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> arr[i];
  int mov = moveee(n);
  if (mov == -1) return cout << -1 << endl, 0;
  for (int i = mov; i >= 1; i--) {
    int a = i, b = i << 1, c = b + 1;
    int maxi = max(arr[b], arr[c]);
    arr[a] = max(0, arr[a] - maxi);
    arr[b] = max(0, arr[b] - maxi);
    arr[c] = max(0, arr[c] - maxi);
    cnt += maxi;
  }
  if (arr[1] > 0) cnt += arr[1];
  cout << cnt << endl;
  return 0;
}
