#include "bits/stdc++.h"
#define _USE_MATH_DEFINES
#include <math.h>

using namespace std;

using LL = long long;
using VLL = std::vector<LL>;
using VVLL = std::vector<VLL>;
using VVVLL = std::vector<VVLL>;
using LD = long double;
using VLD = std::vector<LD>;
using VVLD = std::vector<VLD>;
using VVVLD = std::vector<VVLD>;
using BL = bool;
using VBL = std::vector<BL>;
using VVBL = std::vector<VBL>;
using VVVBL = std::vector<VVBL>;
using SS = string;
using VSS = std::vector<SS>;

#define REP(i,n) for(LL (i)=0;(i)<(LL)(n);(i)++)
#define REPM(i,n,m) for(LL (i)=m;(i)<(LL)(n);(i)++)
#define REPI(i,n) for(LL (i)=(n-1);(i)>=(LL)(0);(i)--)
#define ALL(v) v.begin(), v.end()
#define PRINTLN(v) {LL i=0; for(auto (a): v ){   cerr <<setw(2) <<i << " : "<<  a << endl; i++;}}

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; }
template<class T> void bye(T a){cout << a << '\n'; exit(0);} 
template<class T1,class T2> void bye(T1 a,T2 b){cout << a <<" " << b << '\n'; exit(0);} 
template<class T1,class T2,class T3> void bye(T1 a,T2 b,T3 c){cout << a <<" " << b <<" " << c << '\n'; exit(0);} 
template<class T> void bye(vector<T> A)
{  
  REP(i,(LL)A.size()){
    cout << A[i];
    if(i!=(LL)A.size()-1){
      cout << " ";
    }
  }
  cout << endl;
  exit(0);
} 


#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)

// vector
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
    for (T &x : vec) is >> x;
    return is;
}
// pair
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}
// vector<string>
ostream &operator<<(ostream &os, const vector<string> &vec) {
  
  LL N = to_string((LL)vec.size()).length();
  string head;
  REP(i,N+2){ head += "-";}
  REP(i,40){ head += to_string(i%10);}
  os << setw(N) << head << endl;
   
  for (LL i = 0; i < (LL)vec.size(); i++) {
    os << setw(N) << i << ": " << vec[i] << endl;
  } 
  return os;
}
// vector<LL>
ostream &operator<<(ostream &os, const vector<LL> &vec) {
  LL N = to_string((LL)vec.size()).length();
  for (LL i = 0; i < (LL)vec.size(); i++) {
    os << setw(N) << i << ": " <<vec[i] << endl;
  } 
  return os;
}
// vector<LD>
ostream &operator<<(ostream &os, const vector<LD> &vec) {
  LL N = to_string((LL)vec.size()).length();
  for (LL i = 0; i < (LL)vec.size(); i++) {
    os << setw(N) << i << ": " <<vec[i] << endl;
    //os <<vec[i] << endl;
  } 
  return os;
}
// VVLL
ostream &operator<<(ostream &os, const vector<vector<LL>> &mat) {
  LL keta = 1;
  REP(i,mat.size()){
    REP(j,mat[i].size()){
      LL x = mat[i][j];
      chmax(keta,(LL)to_string(x).length());
    }
  }
  LL N = to_string((LL)mat.size()).length();
  REP(i,mat.size()){
    os << setw(N) << i << ": " ;
    REP(j,mat[i].size()){
      LL x = mat[i][j];
      os << setw(keta+1) <<x;
    }
    os << endl;
  }  
  return os;
}

// vector
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &vec) {

  for (LL i = 0; i < (LL)vec.size(); i++) {
    //os << setw(3) <<i <<" : "<< setw(5) << vec[i] << endl;
    os << setw(5) << vec[i];
  } 
  os << endl;
  return os;
}
// map
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
    os << "{";
    repi(itr, map_var) {
        os << *itr;
        itr++;
        if (itr != map_var.end()) os << ", ";
        itr--;
    }
    os << "}";
    return os;
}
// set
template <typename T>
ostream &operator<<(ostream &os, set<T> &set_var) {
    os << "{";
    repi(itr, set_var) {
        os << *itr;
        itr++;
        if (itr != set_var.end()) os << ", ";
        itr--;
    }
    os << "}";
    
  return os;
}

#define DUMPOUT cerr
#define DUMPCOLOR ("\033[36m")

void dump_func() {
    DUMPOUT << endl;
}
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&... tail) {
    DUMPOUT << DUMPCOLOR;
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) {
        DUMPOUT << ", ";
    }
    dump_func(std::move(tail)...);
    DUMPOUT << "\033[m";
}
#ifdef DEBUG_
#define DEB
#define dump(...)                                                              \
    DUMPOUT << DUMPCOLOR                                                      \
            << "" << string(#__VA_ARGS__) << ": "                            \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
            << endl                                                            \
            << "\033[m"                                                        \
            << "",                                                         \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

struct Fast{
  Fast(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(std::numeric_limits<double>::max_digits10);
  }
} fast;


//template<class T=LL> inline vector<T> cinv(LL N){ vector<T> v(N); REP(i, N)cin>>v[i]; return move(v);}
//template<class T=LL> inline vector<T> cinv(LL N){ vector<T> v(N); REP(i, N)cin>>v[i]; return move(v);}
//#define PRLLSP(v) for(auto (a): v ) {cerr << a << " ";} cerr << endl


template<class T> 
void PRINTSP(vector<T> v, size_t w=3){
  for(auto (a): v ) {cerr <<setw(w)<< a << " ";} cerr << endl;
} 


struct mll{
  static LL MOD;
  LL val;
  mll(LL v = 0): val(v % MOD){ if (val < 0) val += MOD; }
  mll operator - () const { return -val; }
  mll operator + (const mll &b) const { return val + b.val; }
  mll operator - (const mll &b) const { return val - b.val; }
  mll operator * (const mll &b) const { return val * b.val; }
  mll operator / (const mll &b) const { return mll(*this) /= b; }
  mll operator + (LL b) const { return *this + mll(b); }
  mll operator - (LL b) const { return *this - mll(b); }
  mll operator * (LL b) const { return *this * mll(b); }
  friend mll operator + (LL a, const mll &b) { return b + a; }
  friend mll operator - (LL a, const mll &b) { return -b + a; }
  friend mll operator * (LL a, const mll &b) { return b * a; }
  mll &operator += (const mll &b) { val=(val+b.val)%MOD; return *this; }
  mll &operator -= (const mll &b) { val=(val+MOD-b.val)%MOD; return *this; }
  mll &operator *= (const mll &b) { val=(val*b.val)%MOD; return *this; }
  mll &operator /= (const mll &b) {
    LL c=b.val, d=MOD, u=1, v=0;
    while (d){
      LL t = c / d;
      c -= t * d; swap(c, d);
      u -= t * v; swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0) val += MOD;
    return *this;
  }
  mll &operator += (LL b) { return *this += mll(b); }
  mll &operator -= (LL b) { return *this -= mll(b); }
  mll &operator *= (LL b) { return *this *= mll(b); }
  mll &operator /= (LL b) { return *this /= mll(b); }
  bool operator == (const mll &b) { return val == b.val; }
  bool operator != (const mll &b) { return val != b.val; }
  bool operator == (LL b) { return *this == mll(b); }
  bool operator != (LL b) { return *this != mll(b); }
  friend bool operator == (LL a, const mll &b) { return mll(a) == b.val; }
  friend bool operator != (LL a, const mll &b) { return mll(a) != b.val; }
  friend ostream &operator << (ostream &os, const mll &a) { return os << a.val; }
  friend istream &operator >> (istream &is, mll &a) { return is >> a.val; }
  static mll Combination(LL a, LL b){
    chmin(b, a-b);
    if (b<0) return mll(0);
    mll c = 1;
    REP(i, b) c *= a-i;
    REP(i, b) c /= i+1;
    return c;
  }
  static vector<mll> Combination(LL N){
    if (N<0)  return vector<mll>(1);
    mll c = 1;
    vector<mll> K(N+1);
    K[0]=1;
    REPM(i,N+1,1){
      c *= i;
      K[i] = c;
    }
    vector<mll> C(N+1);
    C[0]=1;
    REPM(k,N+1,1){
      C[k] = K[N]/(K[N-k]*K[k]);
    }
    return C;
  }
  static vector<mll> pows(LL N, LL M){
    if (N<0)  return vector<mll>(1);
    vector<mll> P(M+1);
    P[0]=1;
    REPM(i,M+1,1){
      P[i] = P[i-1]*N;
    }
    return P;
  }
  static mll Kumiawase(LL a, LL b){
    chmin(b, a-b);
    if (b<0) return mll(0);
    return Junretu(a,b)/Kaijou(b);
  }
  static mll Kaijou(LL a){
    if (a<0)  return mll(0);
    mll c = 1;
    for(LL i=1;i<=a;i++){
      c *= i;
    }
    return c;
  }
  static mll Junretu(LL a, LL b){
    if(a<b){
      return mll(0);
    }
    mll c = 1;
    for(LL i=a;i>a-b;i--){
      c *= i;
    }

    return c;
  }
  static mll _Junretu(LL a, LL b){
    if(a<b){
      return mll(0);
    }
    return Kaijou(a)/(Kaijou(a-b));
  }
  LL get(){
    return val;
  }
};
LL mll::MOD = (LL)(1e9 + 7);// 998244353LL;//(LL)(1e9 + 7);
using vmll = std::vector<mll>;
using vvmll = std::vector<vmll>;
using vvvmll = std::vector<vvmll>;
using vvvvmll = std::vector<vvvmll>;

// a^n mod を計算する
LL modpow(LL a, LL n, LL mod) {
    long long res = 1;
    while (n > 0) {
        if (n & 1) res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}

template<typename TTT>
vector<LL> arg_sort(vector<TTT> A, bool ascend = true) {
  vector<LL> index(A.size());
  iota(index.begin(), index.end(), 0);
  if (ascend) {
    std::sort(index.begin(), index.end(), [&A](TTT i1, TTT i2) {
      return A[i1] < A[i2];
      });
  }
  else {
    std::sort(index.begin(), index.end(), [&A](TTT i1, TTT i2) {
      return A[i1] > A[i2];
      });
  }

  return index;
}

template<typename TTT>
vector<TTT> permute(vector<TTT> src, VLL idx){
  vector<TTT> dst(src.size());
  REP(i,src.size()){
    dst[i] = src[idx[i]];
  }
  return move(dst);
}


template<typename _Iterator, typename _Compare>
LL num_of_ika(_Iterator _first, _Iterator _last, _Compare key){
  return (LL)(upper_bound(_first,_last,key)-_first);
}
template<typename _Iterator, typename _Compare>
LL num_of_ookii(_Iterator _first, _Iterator _last, _Compare key){
  return (LL)(_last-upper_bound(_first,_last,key));
}
template<typename _Iterator, typename _Compare>
LL num_of_chisai(_Iterator _first, _Iterator _last, _Compare key){
  return (LL)(lower_bound(_first,_last,key) - _first);
}
template<typename _Iterator, typename _Compare>
LL num_of_ijou(_Iterator _first, _Iterator _last, _Compare key){
  return (LL)(_last-lower_bound(_first,_last,key));
}
template<typename _Iterator, typename _Compare>
LL num_of_onaji(_Iterator _first, _Iterator _last, _Compare key){
  return (LL)(upper_bound(_first,_last,key)-lower_bound(_first,_last,key));
}





#define cmax(a,b) {if(a<b){a=b;}}
#define cmin(a,b) {if(a>b){a=b;}}
#define EACH(a,A) for(auto a: A)
//#define cmax(a,b) a = (a>b ? a:b)
//#define cmin(a,b) a = (a<b ? a:b) 
//cin,cout高速化のおまじない＋桁数指定
inline VLL cinvll(LL N, LL minus = 0){
  VLL A(N);
  REP(i,N){
    cin >> A[i];
    A[i] -= minus;
  }
  return move(A);
}
inline VVLL zerosll(LL H, LL W, LL val=0){
  VVLL A(H,VLL(W,val));
  return move(A);
}
inline VVVLL zerosll3(LL H, LL W,LL C, LL val=0){
  VVVLL A(H,VVLL(W,VLL(C,val)));
  return move(A);
}


bool isPrime(LL N)
{
  if (N < 2) {
    return false;
  }
  else if (N == 2) {
    return true;
  }
  else if (N % 2 == 0) {
    return false;
  }

  LD sqrtNum = sqrt(N);
  for (LL i = 3; i <= sqrtNum; i += 2)
  {
    if (N % i == 0)
    {
      return false;
    }
  }
  return true;
}


#if 1

map< LL, LL > prime_factor(LL n) {
  map< LL,LL > ret;
  for(LL i = 2; i * i <= n; i++) {
    while(n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if(n != 1) ret[n] = 1;
  return ret;
}

LL GCD(LL m, LL n)
{
  if(m < n) {
    swap(m, n);
  }

  while(n != 0) {
    LL n_new = m % n;
    m = n;
    n = n_new;
  }
  return m;
}
LL LCM(LL x, LL y) { return x / GCD(x, y) * y; }

#endif

struct Furui{
  //! 1からnまでを素数を求める
  //! isPrime：素数判定
  //! factor：素因数分解した結果をvector<pair<LL,LL>>で返す
  //! factorList：素因数分解した結果をVLLで返す
  LL n;
  VLL f;
  VLL prime;

  Furui(LL n=1):n(n),f(n+1){
    f[0]=-1;
    f[1]=-1;
    for( LL i=2;i<=n;i++){
      if(f[i]){
        continue;
      }
      prime.push_back(i);
      for(LL j=i;j<=n; j+=i){
        if(f[j]==0){
          f[j] = i;
        }
      }
    }
  }
  bool isPrime(LL x){
    if(x<=1){
      return false;
    }
    return (f[x]==x);
  }
  VLL factor_vector(LL x){
    VLL res;
    if( x>=2 and x<=n){
      while( x!=1){
        res.push_back(f[x]);
        x /=f[x];
      }
    }
    return res;
  }
  vector<pair<LL,LL>> factor_pair(LL x){
    VLL factors = factor_vector(x);
    vector<pair<LL,LL>> P;
    if(factors.size()==0){
      return P;
    }
    P.emplace_back(factors[0],0);
    for(auto p : factors){
      if(P.back().first==p){
        P.back().second++;
      }else{
        P.emplace_back(p,1);
      }
    }
    return P;
  }
  map<LL,LL> factor_map(LL x){
    VLL factors = factor_vector(x);
    map<LL,LL> P;
    if(factors.size()==0){
      return P;
    }
    
    for(auto p : factors){
      P[p]+=1;  
    }
    return P;
  }
};

template< typename T >
struct edge {
  int src, to;
  T cost;
  edge(int to, T cost) : src(-1), to(to), cost(cost) {}
  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};
template< typename T >
using Edges = vector< edge< T > >;
template< typename T >
using WeightedGraph = vector< Edges< T > >;
using UnWeightedGraph = vector< vector< int > >;
template< typename T >
using Matrix = vector< vector< T > >;

template< typename T >
void warshall_floyd(Matrix< T > &g, T INF) {
  for(int k = 0; k < g.size(); k++) {
    for(int i = 0; i < g.size(); i++) {
      for(int j = 0; j < g.size(); j++) {
        if(g[i][k] == INF || g[k][j] == INF) continue;
        g[i][j] = min(g[i][j], g[i][k] + g[k][j]);
      }
    }
  }
}





bool isOK(LL mid)
{
  return true; 
}
// 汎用的な二分探索のテンプレ
LL binary_search() {
  LL ng = 0 ;
  LL ok = 100000;

  /* ok と ng のどちらが大きいかわからないことを考慮 */
  while (abs(ok - ng) > 1) {
      LL mid = (ok + ng) / 2;

      if (isOK(mid)) ok = mid;
      else ng = mid;
  }
  return ok;
}

LL powll(LL a,LL b){
  LL c = 1LL;
  REP(i,b){
    c *= a;
  }
  return c;
}

LL knapsack(vector<LL> v,vector<LL> w,LL W) {
  LL N = (LL)v.size();
  vector<LL> dp(W+1,0);
  for (LL i=0; i<N; ++i){
    for (LL j=w[i]; j<=W; ++j){
      dp[j] = max(dp[j], dp[j-w[i]] + v[i]);
    }
  }

  return dp[W];
}



LL Combination(LL a, LL b)
{
  chmin(b, a - b);
  if(b < 0)
    return 0;
  LL c = 1;
  REP(i, b) c *= a - i;
  REP(i, b) c /= i + 1;
  return c;
}

bool isKaibun(string S){
  bool ok = true;
  LL N = (LL)S.length();
  REP(i,N){
    if(S[i]!=S[N-i-1]){
      ok = false;
      break;
    }
  }
  return ok;
}


VLL haba_yuusen_kyori(VVLL &V, LL n ){
  LL N = V.size();
  VLL D(N,-1);
  queue<pair<LL,LL>> Q;
  Q.push({n,0});
  while(!Q.empty()){
    auto q = Q.front();
    Q.pop();
    LL no = q.first;
    LL d  = q.second;
    D[no] = d;
    for(auto v : V[no]){
      if(D[v]!=-1){
        continue;
      }
      D[v] = d+1;
      Q.push({v,d+1});
    }    
  }
  return move(D);  
}  

void oira_tua(LL me, LL oya,VVLL &T,VLL &IO){
  IO.push_back(me);
  dump(me,oya);
  for(LL i=0;i<(LL)T[me].size();i++){
    LL ko = T[me][i];
    if(ko==oya){
      continue;
    }
    oira_tua(ko,me, T,IO);
    IO.push_back(me);
  }
}
VVLL cinvll2(LL H,LL W,LL bias = 0){
  VVLL A = zerosll(H,W);
  REP(i,H){
    REP(j,W){
      cin >> A[i][j];
      A[i][j] += bias;
    }
  }
  return move(A);
}    




tuple<VLL,VLL> aho2(LL N){
  VLL A(N),B(N);
  REP(i,N){
    cin >> A[i] >> B[i];
  }
  return(make_tuple(A,B));
}

template<class T=LL> inline vector<T> cinv(LL N,T val=0){ vector<T> v(N); REP(i, N){cin>>v[i]; v[i]+=val;} return move(v);}
template<class T=LL> inline vector<vector<T>> zeros(LL H, LL W, T val=0){
  vector<vector<T>> A(H,vector<T>(W,val));
  return move(A);
}
template<class T=LL> inline vector<vector<vector<T>>> zeros3(LL H, LL W,LL C, T val=0){
  vector<vector<vector<T>>> A(H,vector<vector<T>>(W,vector<T>(C,val)));
  return move(A);
}
VLL::iterator lower_bound_right(
  VLL::iterator begin, VLL::iterator end, LL v){
  auto it_l = lower_bound(begin,end,v);
  if(it_l==end){
    return end;
  }
  auto it_r = upper_bound(it_l,end,*it_l);
  it_r--;
  return it_r;
}



void funcA(){
  LL H1,M1,H2,M2,K;
  cin >> H1>>M1>>H2>>M2>>K;

  LL A = (60*(H2-H1) + M2-M1)-K;
  bye(A);


}

void funcB(){
  SS T;
  cin >> T;
  REP(i,T.length()){
    if(T[i]=='?'){
      T[i] = 'D';
    }
  }  
  bye(T);
}
void func1(LL N, VLL &A){
  

}


void func(){
  LL N;
  cin >> N;
  N += 1;
  VLL A = cinv(N);
  if(N==1 and A[0]==1){
    bye(1);
  }
  if(N==1 and A[0]==0){
    bye(-1);
  }
  reverse(ALL(A));
  dump(A);
  VVLL LH = zeros(N,2);
  LH[0][0] = A[0];
  LH[0][1] = A[0];
  REPM(i,N,1){
    LH[i][0] = A[i] + (LH[i-1][0]+1)/2;
    LH[i][1] = A[i] + (LH[i-1][1]);    
  }
  reverse(ALL(A));
  reverse(ALL(LH));
  dump(A);
  dump(LH);
  if(not (LH[0][0]<=1 and 1<=LH[0][1])){
    bye(-1);
  }
  VLL B(N);
  B[0] = 1;
  REPM(i,N,1){
    LL a = B[i-1]-A[i-1];
    B[i] = min(2*a,LH[i][1]);
  }
  dump(B);
  LL cnt = accumulate(ALL(B),0ll);
  bye(cnt);
}
int main()
{
  dump("start");
  func();
  return 0;
}









Measured memory_bytes: 10640