/* Aa^~ kokoro ga pyonpyon suru n jaa^~
// ZZZXXkXkkkZ!``` ``` ``` ``` ``` ``` ``` ``` ``` ``` ``` ```?Wfpppbpbbpbbpbbbkbkk
// ppbbbpbbpVr`` `` ` ` ` ` ```` `` ` ` ``  ` ` ` ` ` ` ` ` `  dppbbkkkkkkkkkkqkqkk
// HkqqqqqkkWr`` ` ` ``` ``` `?G, ` ` ``.JC!```` ` ` `` `` ````(Wpbkkkkkkkkqkkkkkqk
// mmmmmqqqqpr` `` `` ```````.+zT=`` `` 7TO-.```````` `` `` ```(yppbkkkkkkkkkkkkkkk
// ggmgmmqqqH$ ``````````....````` `  ````````.`````` `` ``````.yfpppbbbbkkkkqqqqqH
// gmmmmmqqqkW<```` `````...`````  .,.`  ````....````` ``````` (Wbqqmgmmgmggggggggg
// qmmmqqqqkkWk.``````````````````` ;:<`` `````.`````````````-_<-?WHHqmmmmmmgmmgggg
// @@@@@@@gggHH6- ``````````````` `` _ `` ```````````````` ._~~_.`-?Wkqmmmmmmmggg@g
// @@@@g@gggHY~.-<_- `````````````````````````````````` ._~~(<-``.`.(WHqqqmmggggmmm
// @@g@gggHH=.`..._<-___..```````````````````````. .-_~~~_(!``-.``.`` OHHWUWHmqHWXW
// gggggmqK1.``..~.. _<<+-(____..  ````````  ..__~~_((<<!.`.``` .``.`` j0C1XUHmHIdW
// ggmmqH0!,_``.>`````` _<<;<v<<<++((((((((((<<<<<<~_. (-.``~``.>..```  jOuWHHqHIdH
// gmmqkW!`(_ J>`    `` ` _~<`_~~~~<<<<<<<~~__````````` ?1. ._`(__```    zXWHg@HkXH
// gHHWS{``(lJ<!``.``.```(:+>`._`````.` <..`` - ``. `   _ ?&._.I`_`````` .XyVfppppW
// HHHSv``.(X:_..... _..(;+<!.(<..-.....-.-_..+_`..<.`.`..`_IJd}  .`..````jqg@@@@@@
// XHWZ{..<Jk~!.`.. (<.-(+>(_.(1.(_..`.`.`.<_.+<_..<<-..._..-zy>.`_`...```.WH@HHHHH
// bkWt~.-jCz(_..`.(+<.(;< ._-<=_(<_..-....(_.<1<..(<<.`._..-JUS-._.`...```dHmH9VUH
// WUUO..(f.(c...__+z<-(+~` _-+<_(><..__.`.(<._.z_.(1;_..__.(C(zT-(..`...``(WHR<+Xk
// kkkk._(_.->..._(z;:_><.._>_+_<(1>_._<...(v<<.(<.(+z<..-_(Z~_<_j+_..`...`(WHKz1ZW
// @@gR._+_..~..-<+z<<?<>```_.<_.(+1><_;_..(1_:`.<<??1z--(+Z!..<_.j<....`..(bgHAAQX
// @@mR.(j:..~.._<z!`.(>~``` ~(_.(+<1><><_.(((_`.<__`.<_.(X>...<_.(<.....`.JUWWWyWW
// @gmH_(zl..(.._+>```<+_````.~>``(+.<?>>_._(<```(<<``(__<>....<.._<.......dXkkkHHH
// mmqHl(dk_.(_.-=~`.`.1-..._~-1.``_:`(??<_~(`.--.&_.`.<(;<...._.._<..`..._Xg@@@@@@
// qHkpk(dX<.(;..j_```.(((JJ&a&-~``````.1<_```-(((e+.-(/`(>...._..(<......(Wmggg@@g
// HVHbWcz><__+_.(_.(dWWHHH@HHc~````````.+~`` (jHMMMHHHm&.?..._<..(<_..._.(WqqHHmHg
// 0>vWWkzZwl~<o.__`__~X@@HM@Hb ```.`.``. ```` d@@HHH@@K?76...(<..(<_...(_(ppWWWWHq
// X0XWHKXXw$<(z<.( `` WHHMHHHH_``````````````.WHHMNMHHH_`(...(<_.(z_..._<(fWVC174W
// XuXWHHWWz>__+z+.!`..??CZYCOX_`````````````.`~.OvTUZUS_`~.._+?_.(_~_.._zjO=1+~+jy
// kkkkkkkkX:._<z=1(_`` << ``->``.``.``.``.```` ?<`` (v!`._..(??_.(1._.._=dUOOzzzwX
// @@@@@@@@H<...1O=v<_...__ -_````````````````.`` `` ~.`` :.~+=?~.(;_(...jdQQQQQkkk
// H@@@@@@@H~...(==>.~~~~~....`.`````````.`````.`........->.(===~~<<.(...(dg@@@@@@@
// @@@H@@HHH_.__(=l>~.~~~~~....``.``.``.```..`......~~~~~(<_+=l=~_<.->..~_dqggggg@g
// @H@@@@MHH_._<(=l>...........```````````````.`...~~~~~~+<(=lz=~((j=z_..~jWqmmgggm
// @@H@@HHWH_._<(lll-.......```.````.``.`..`````........_z<+llZz~(lOO=<...(VYUUUW9Y
// @@HMMHWZf>~_=:=llw+.`````````.`.```__~~_``.`````.....(z+llOOz_zllOlz~..~<<1+dW>_
// MMM#MHHWXl~_=>1ltwOl&.`.``.`````.``````````````.````.(llttwtz(OltwOz<..__zwOwwOz
// HM#HMHUUI<._1z+ttOZttlt&....``.``.`.````.``...``...(zZtttOktzjttttwlz_._<(Xkkkkk
// HHHmHSZu:(_~+OztttXtttOZZttO+-..............-(+ztOttwttttd0tOZttttwOl<~.(_dMMHHH
// rvuuXuuI~~<~(uttttwvOwwwkQQHMMHHHHHHHHHMMMNmgey?OwwwrtttwXOtwttttttXtO-~.((wZyyy
// HHHHHHK>(~(-(dOrtrrl(QgMHMMMHHHHHHHHHHHHHHHH##HMNkX0rrrrXXrd%`` (Ctwwtz_~.<(Wg@H
// NNNNNHD(~(zo~zXrrrQdHHMMNMHHHHHHHHHHHHHHHHHHHHHH##HNmyrdKkwZ ` _``-zwrt1~~_<(MNM
// MMMMM#<<_jwr:(Z4QHHMMHMHHHHHHHHHHHHHHHHHHHHHHHHHHHH###NHSXZ>` ~````.OXtt>~._<?MM
*/

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

using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;
using LL = long long;
using VL = vector<LL>;
using VVL = vector<VL>;
using PLL = pair<LL, LL>;
using VS = vector<string>;

#define ALL(a)  begin((a)),end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define SZ(a) int((a).size())
#define SORT(c) sort(ALL((c)))
#define RSORT(c) sort(RALL((c)))
#define UNIQ(c) (c).erase(unique(ALL((c))), end((c)))

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

#define FF first
#define SS second

template<class S, class T>
istream& operator>>(istream& is, pair<S,T>& p){
  return is >> p.FF >> p.SS;
}
template<class T>
istream& operator>>(istream& is, vector<T>& xs){
  for(auto& x: xs)
	is >> x;
  return is;
}
template<class S, class T>
ostream& operator<<(ostream& os, const pair<S,T>& p){
  return os << p.FF << " " << p.SS;
}
template<class T>
ostream& operator<<(ostream& os, const vector<T>& xs){
  for(unsigned int i=0;i<xs.size();++i)
	os << (i?" ":"") << xs[i];
  return os;
}
template<class T>
void maxi(T& x, T y){
  if(x < y) x = y;
}
template<class T>
void mini(T& x, T y){
  if(x > y) x = y;
}
void debug(istringstream&){}
template <char sep=',', class Head, class... Tail>
void debug(istringstream& iss, Head&& head, Tail&&... tail)
{
  string name;
  getline(iss, name, ',');
  cout << sep << name << "=" << head;
  debug(iss, forward<Tail>(tail)...);
}

#ifdef PYONPOI
#define DEBUG(...) \
  do{ \
    istringstream ss(#__VA_ARGS__); \
    debug<' '>(ss, __VA_ARGS__); \
    cout<<endl; \
  }while(0)
#else
#define DEBUG
#endif
void init_io(int argc, char* argv[]){
  cin.tie(0);
  ios_base::sync_with_stdio(false);
#ifdef PYONPOI
  if(argc > 1) {
    ifstream *ifs = new ifstream();
    ifs->open(argv[1]);
    cin.rdbuf(ifs->rdbuf());
  }
#endif
}


const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1e9+7;
const LL INF = 1e18;

int main(int argc, char* argv[]){
  init_io(argc, argv);

  int N;
  cin >> N;

  VL xs(N+1);
  cin >> xs;

  bool ok = true;
  if(xs[0] > 1){
    cout << -1 << endl;
    return 0;
  }

  VL rest(N+1);
  rest[0] = 1 - xs[0];
  FOR(i,1,N+1){
    rest[i] = min(INF, rest[i-1] * 2 - xs[i]);
    if (rest[i] < 0) ok = false;
  }

  if (rest[N] < 0) ok = false;
  VL ns(N+1);
  ns[N] = 0;
  for(int i=N-1;i>=0;--i){
    ns[i] = min(rest[i], xs[i+1] + ns[i+1]);
  }

  DEBUG(rest);
  DEBUG(ns);

  LL ans = 0;
  REP(i,N+1) ans += ns[i] + xs[i];

  if(!ok) ans = -1;
  cout << ans << endl;

  return 0;
}


Measured memory_bytes: 5576