#include <bits/stdc++.h>
using namespace std;

#define rep(i,s,n) for (int i = (ll)s; i < (ll)n; i++)
#define rrep(i,n,e) for (int i = (ll)n; i > (ll)e; i--)
#define ll long long
#define ld long double
#define pb push_back
#define eb emplace_back
#define All(x) x.begin(), x.end()
#define Range(x, i, j) x.begin() + i, x.begin() + j
#define lbidx(x, y) lower_bound(x.begin(), x.end(), y) - x.begin()
#define ubidx(x, y) upper_bound(x.begin(), x.end(), y) - x.begin()
#define llbidx(x, y, z) lower_bound(x.begin(), x.end(), z) - lower_bound(x.begin(), x.end(), y) // 二要素間の距離
// #define M_PI 3.14159265358979323846 // CFでは定義しておく必要あり
#define deg2rad(deg) ((((double)deg)/((double)360)*2*M_PI))
#define rad2deg(rad) ((((double)rad)/(double)2/M_PI)*(double)360)
#define Find(set, element) set.find(element) != set.end()
#define Decimal(x) cout << fixed << setprecision(10) << x << endl; // 小数点を10桁まで表示
// debug用
#define PrintVec(x) for (auto elementPrintVec: x) { cout << elementPrintVec << " "; } cout << "\n";
#define debug(x) cerr << #x << ": " << (x) << "\n";
#define endl "\n"
// gcj print用
#define Case(x) printf("Case #%d: ", x);

typedef pair<int, int> PI;
typedef pair<ll, ll> PLL;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<vector<vector<int>>> vvvi;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
typedef vector<vector<vector<int>>> vvvl;
typedef vector<PI> vpi;
typedef vector<vector<PI>> vvpi;
typedef vector<vector<vector<PI>>> vvvpi;
typedef vector<PLL> vpl;
typedef vector<vector<PLL>> vvpl;
typedef vector<vector<vector<PLL>>> vvvpl;

const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;

int POWINT(int x, int n) {
  int ret = 1;
  while (n > 0) {
    if (n & 1) ret *= x;
    x *= x;
    n >>= 1;
  }
  return ret;
};

ll POWLL(ll x, int n) {
  ll ret = 1;
  while (n > 0) {
    if (n & 1) ret *= x;
    x *= x;
    n >>= 1;
  }
  return ret;
};

template<class T>
inline bool chmax(T &a, T b) {
  if(a < b) {
    a = b;
    return true;
  }
  return false;
};

template<class T>
inline bool chmin(T &a, T b) {
  if(a > b) {
    a = b;
    return true;
  }
  return false;
};

void print() {
  cout << endl;
};

template <class T>
void print(vector<T> &vec) {
  for (auto& a : vec) {
    cout << a;
    if (&a != &vec.back()) cout << " ";
  }
  cout << endl;
};

template <class T>
void print(vector<T> &vec, ll k){
   ll n = vec.size();
   k = min(k, n);
   rep(i, 0, k-1) cout << vec[i] << " ";
   cout << vec[k-1] << endl;
};

template <class T>
void print(vector<vector<T>> &df) {
  for (auto& vec : df) {
    print(vec);
  }
};

template<class T, class U>
void print(pair<T,U> &p){
  cout << p.first << " " << p.second << "\n";
};

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  ll N; cin >> N;
  vl A(N+1), B(N+1), sumA(N+2);
  rep(i, 0, N+1) cin >> A[i];
  sumA[N] = A[N];
  rrep(i, N-1, -1) {
    sumA[i] = A[i] + sumA[i+1];
  }
  bool ok = true;
  if (A[0] >= 2) {
    ok = false;
  } else {
    B[0] = 1 - A[0];
  }

  rep(i, 1, N+1) {
    B[i] = min<ll>(B[i-1]*2-A[i], sumA[i+1]);
    if (B[i] < 0) ok = false;
  }

  ll ans = 0;
  rep(i, 0, N+1) {
    ans += A[i] + B[i];
  }

  if (ok) {
    cout << ans << endl;
  } else {
    cout << -1 << endl;
  }

  return 0;
};


Measured memory_bytes: 5560