#line 2 "header.hpp"

//%snippet.set('header')%
//%snippet.fold()%
#ifndef HEADER_H
#define HEADER_H

// template version 2.0
using namespace std;
#include <bits/stdc++.h>

// varibable settings
const long long INF = 1e18;
template <class T> constexpr T inf = numeric_limits<T>::max() / 2.1;

#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (ll i = (ll)(a); i < (ll)(b); ++i)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)
#define _rrep(i, n) rrepi(i, 0, n)
#define rrepi(i, a, b) for (ll i = (ll)((b)-1); i >= (ll)(a); --i)
#define r_rep(...) _overload3(__VA_ARGS__, rrepi, _rrep, )(__VA_ARGS__)
#define each(i, a) for (auto &&i : a)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(...) make_tuple(__VA_ARGS__)
#define ub upper_bound
#define lb lower_bound
#define lpos(A, x) (lower_bound(all(A), x) - A.begin())
#define upos(A, x) (upper_bound(all(A), x) - A.begin())
template <class T> inline void chmax(T &a, const T &b) { if ((a) < (b)) (a) = (b); }
template <class T> inline void chmin(T &a, const T &b) { if ((a) > (b)) (a) = (b); }
template <typename X, typename T> auto make_table(X x, T a) { return vector<T>(x, a); }
template <typename X, typename Y, typename Z, typename... Zs> auto make_table(X x, Y y, Z z, Zs... zs) { auto cont = make_table(y, z, zs...); return vector<decltype(cont)>(x, cont); }

#define divceil(a, b) ((a) + (b)-1) / (b)
#define is_in(x, a, b) ((a) <= (x) && (x) < (b))
#define uni(x) sort(all(x)); x.erase(unique(all(x)), x.end())
#define slice(l, r) substr(l, r - l)

typedef long long ll;
typedef long double ld;
using vl = vector<ll>;
using vvl = vector<vl>;
using pll = pair<ll, ll>;

template <typename T>
using PQ = priority_queue<T, vector<T>, greater<T>>;
void check_input() { assert(cin.eof() == 0); int tmp; cin >> tmp; assert(cin.eof() == 1); }

#if defined(PCM) || defined(LOCAL)
#else
#define dump(...) ;
#define dump_1d(...) ;
#define dump_2d(...) ;
#define cerrendl ;
#endif

#endif /* HEADER_H */
//%snippet.end()%
#line 2 "solve.cpp"
template<class T=ll> using vec = vector<T>;
struct Fast { Fast() { std::cin.tie(0); ios::sync_with_stdio(false); } } fast;


int solve() {
    ll n;cin>>n;/*{{{*/
    vector<ll> a(n+1);
    ll now = 1;
    rep(i, n+1) {
        cin>>a[i]; 
        if (a[i]>now) {
            cout << -1 << endl;
            return 0;
        }
        if (now <= 1e9) { now *= 2; }
    }

    if (n==0){
        if (a[0]==1){
            cout << 1 << endl;
            return 0;
        }
        else{
            cout << -1 << endl;
            return 0;
        }
    }/*}}}*/

    dump(a);
    if (a[0]!=0){
        cout << -1 << endl;
        return 0;
    }

    vl pu2(n+1), pd2(n+1);
    r_rep(i, 0, n){
        pu2[i] = a[i+1] + pu2[i+1];
        pd2[i] = divceil(pd2[i+1]+a[i+1], 2);
    }
    dump(pu2);
    dump(pd2);

    vl pu(n+1), pd(n+1);
    pu[0] = 1;
    pd[0] = 1;
    rep(i, 1, n+1){
        if (pu[i-1] )
        pu[i] = pu[i-1] * 2 - a[i];
        chmin(pu[i], pu2[i]);
        if (pu[i]<0){
            // TODO
            // assert(false);
            cout << -1 << endl;
            return 0;
        }
        pd[i] = max(pd[i-1] - a[i], 0LL);
    }
    dump(pu);
    dump(pd);

    ll ans = 0;
    rep(i, 0, n+1){
        ll d = max(pd[i], pd2[i]);
        ll u = min(pu[i], pu2[i]);
        if (d<=u){
            ans += u + a[i];
        }
        else {
            cout << -1 << endl;
            return 0;
        }
    }
    cout << ans << endl;

    return 0; 
}


int main(){/*{{{*/
    solve();
    return 0;
}/*}}}*/


Measured memory_bytes: 7160