#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <chrono>
#include <climits>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <unordered_map>
#include <unordered_set>
#include <vector>
using namespace std;

template <class T1, class T10>
ostream& operator << (ostream& out, const pair <T1, T10> p)
{
	out << '(' << p.first << ',' << p.second << ')';
	return out;
}

template <class T1, class T10>
istream& operator >> (istream& in, pair<T1, T10> &p)
{
	in >> p.first >> p.second;
	return in;
}

template <class T>
istream& operator >> (istream& in, vector<T> &v)
{
	for (T &x : v)
		in >> x;
	return in;
}

template <class T>
ostream& operator << (ostream& out, const vector<vector<T>> &v)
{
	for (const vector<T> &x : v)
		out << x << '\n';
	return out;
}

template <class T>
ostream& operator << (ostream& out, const vector<T> &v)
{
	for (const T &x : v)
		out << x << ' ';
	return out;
}

long long gcd (long long a, long long b)
{
	if (b > a)
		swap(a, b);
	return (b ? gcd(b, a % b) : a);
}

using ll   = long long;
using pii  = pair<int, int>;
using pll  = pair<long long, long long>;
using tiii = pair<pair<int, int>, int>;
using vi   = vector<int>;
using vl   = vector<long long>;
using vvi  = vector<vector<int>>;
using vvl  = vector<vector<long long>>;

#define F          first
#define S          second
#define First      first.first
#define Second     first.second
#define Third      second
#define mp         make_pair
#define rep(i,a,b) for (int i = (a); i < (b); i++)
#define per(i,b,a) for (int i = (b); i > (a); i--)
#define all(x)     x.begin(), x.end()
#define ret(x)     return cout << x, 0;
#define throwex    throw runtime_error ("Found the error.");

const int h = 100000007;

signed main()
{
	ios::sync_with_stdio(false);
	#ifdef ONLINE_JUDGE
	cin.tie(nullptr);
	cout.tie(nullptr);
	cerr.setstate(ios::failbit);
	#endif

	int n;
	cin >> n;
	vi v(n+1);
	cin >> v;
	if(v.front() > 1)
		ret(-1);
	vl vcx(n+1, (LLONG_MAX/10)), vcn(n+1, (LLONG_MIN/10));
	rep(i,0,n+1)
		vcn[i] = v[i];
	vcx[0] = vcn[0] = 1;
	rep(i,1,n+1)
	{
		vcx[i] = min(vcx[i], (vcx[i-1] - v[i-1]) * 2);
		vcn[i] = max(vcn[i], (vcn[i-1] - v[i-1]));
		vcx[i] = max(vcx[i], (ll)(LLONG_MIN/10));
		vcn[i] = min(vcn[i], (ll)(LLONG_MAX/10));
		// cerr << vcn[i] << ' ' << vcx[i] << '\n';
		if(vcx[i] < vcn[i])
			ret(-1);
	}
	// cerr << vcx.back() << ' ' << vcn.back() << ' ' << v.back() << '\n';
	if(vcx.back() < v.back() or vcn.back() > v.back())
		ret(-1);
	vcx.back() = v.back();
	vcn.back() = v.back();
	// cerr << vcx.back() << ' ' << vcn.back() << ' ' << v.back() << '\n';
	per(i,n-1,-1)
	{
		// cerr << vcn << '\n';
		// cerr << vcx << '\n';
		// cerr << vcx[i] << ' ' << vcx[i+1] << '\n';
		vcx[i] = min(vcx[i], vcx[i+1] + v[i]);
		vcn[i] = max(vcn[i], (vcn[i+1] + 1) / 2 + v[i]);
		vcx[i] = max(vcx[i], (ll)(LLONG_MIN/10));
		vcn[i] = min(vcn[i], (ll)(LLONG_MAX/10));
		// cerr << vcn[i] << ' ' << vcx[i] << '\n';
		if(vcx[i] < vcn[i])
			ret(-1);
	}
	ll ans = 0;
	rep(i,0,n+1)
	{
		cerr << vcn[i] << ' ' << vcx[i] << '\n';
		ans += vcx[i];
	}
	cout << ans;
}


Measured memory_bytes: 5312