#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <climits>
#include <cstring>
#include <cassert>


using namespace std;
#define SZ(x) (int)(x.size())
#define REP(i, n) for(int i=0;i<n;++i)
#define FOR(i, a, b) for(int i=a;i<b;++i)
#define RREP(i, n) for(int i=n-1;i>=0;--i)
#define RFOR(i, a, b) for(int i=b-1;i>=a;--i)

#define all(x) begin(x),end(x)
#define dump(x) cerr<<#x<<" = "<<(x)<<endl
#define debug(x) cerr<<#x<<" = "<<(x)<<" (L"<<__LINE__<<")"<< endl;

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using P = pair<int, int>;
const double eps = 1e-8;
const ll MOD = 1000000007;
const int INF = INT_MAX / 2;
const ll LINF = LLONG_MAX / 2;

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

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


template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> p) {
    os << p.first << ":" << p.second;
    return os;
}

template<class T>
ostream &operator<<(ostream &os, const vector<T> &v) {
    os << "[";
    REP(i, SZ(v)) {
        if (i) os << ", ";
        os << v[i];
    }
    return os << "]";
}

// edit

bool cmp(ll l, ll m, ll r){
    return l <= m && m <= r;
}

void solve() {
    int N;
    cin >> N;
    
    N++;
    
    vector<ll> A(N);
    REP(i, N) cin >> A[i];
    
    if(N == 0){
        cout << 0 << endl;
        return;
    }
    
    if(N == 1){
        if(A[0] == 1){
            cout << 1 << endl;
            return;
        }else{
            cout << -1 << endl;
            return;
        }
    }
    
    if(A[0] != 0){
        cout << -1 << endl;
        return;
    }
    
    vector<ll> L(N), R(N);
    for(int i = N - 2; i >= 0; --i){
        L[i] = (A[i + 1] + L[i + 1] + 1) / 2;
        R[i] = A[i + 1] + R[i + 1];
    }
    
    if(!cmp(L[0], 1, R[0])){
        cout << -1 << endl;
        return;
    }
    
    vector<ll> x(N);
    x[0] = 1;
    
    for(int i = 1; i < N; ++i){
        ll left = max(x[i - 1] - A[i], L[i]);
        ll right = min(2 * x[i-1] - A[i], R[i]);
        if(left <= right){
            x[i] = right;
        }else{
            cout << -1 << endl;
            return;
        }
    }
    
    ll ans = 0;
    for(int i = 0; i < N; ++i){
        ans += A[i];
        ans += x[i];
    }
    
    cout << ans << endl;

}


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
//    std::ifstream in("input.txt");
//    std::cin.rdbuf(in.rdbuf());

    solve();


    return 0;
}


Measured memory_bytes: 6048