#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <limits>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <functional>
#include <iomanip>

using namespace std;

#ifdef LOCAL
#define eprintf(...) fprintf(stderr, __VA_ARGS__)
#else
#define eprintf(...) 42
#endif
using ll = long long;
using vll = std::vector<ll>;
using vvll = std::vector<vll>;
using vvvll = std::vector<vvll>;
#define reps(i, S, E) for (ll i = (S); i <= (E); i++)
#define rep(i, N) reps(i, 0, N-1)
#define deps(i, E, S) for (ll i = (E); i >= (S); i--)
#define dep(i, N) deps(i, N-1, 0)
//const ll INF = 1LL << 60;
//const int INF_INT = 1 << 30;
const ll INF = 9223372036854775807ll / 2ll;
const int INF_INT = 400000000;

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; }
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;
	}
};
using vmll = std::vector<mll>;
using vvmll = std::vector<vmll>;
using vvvmll = std::vector<vvmll>;
using vvvvmll = std::vector<vvvmll>;

struct Fast {
	Fast() {
		cin.tie(0);
		ios::sync_with_stdio(false);
		cout << fixed << setprecision(std::numeric_limits<double>::max_digits10);
	}
} fast; //cin,cout高速化のおまじない＋桁数指定

ll mll::MOD = (ll)(1e9 + 7);// 998244353ll;

//--debug--
#ifdef LOCAL
void _debug_out_(string s = "") {
	if (s != "") { eprintf(s.c_str()); }
	else { eprintf("\n"); }
}
void _debug_out_(ll x, string s = "") {
	if (s != "") { eprintf(s.c_str(), x); }
	else { eprintf("%lld\n", x); }
}
void _debug_out_(mll x, string s = "") {
	if (s != "") { eprintf(s.c_str(), x); }
	else { eprintf("%lld\n", x.val); }
}
void _debug_out_(vvll x, string s = "") {
	string t = s;
	if (s == "") {
		ll maxLen = 0;
		rep(i, (ll)x.size()) rep(j, (ll)x[i].size()) {
			ll n = x[i][j] / 10, len = 1;
			while (n > 0) { len++; n /= 10; }
			chmax(maxLen, len);
		}
		t = "%" + to_string(maxLen) + "lld ";
	}
	rep(i, (ll)x.size()) { rep(j, (ll)x[i].size()) { eprintf(t.c_str(), x[i][j]); } eprintf("\n"); }
	eprintf("\n");
}
void _debug_out_(vll x, string s = "") {
	rep(i, (ll)x.size()) { if (s != "") { eprintf(s.c_str(), x[i]); } else { eprintf("%lld ", x[i]); } }
	eprintf("\n");
}
void _debug_out_(vector<bool> x) {
	rep(i, (ll)x.size()) { eprintf("%s", x[i] ? "o" : "x"); }
	eprintf("\n");
}
void _debug_out_(vector<vector<bool>> x) {
	rep(i, (ll)x.size()) { rep(j, (ll)x[i].size()) { eprintf("%s", x[i][j] ? "o" : "x"); } eprintf("\n"); }
	eprintf("\n");
}
void _debug_out_(map<ll, ll> x) {
	auto begin = x.begin(), end = x.end();
	for (auto iter = begin; iter != end; iter++) { eprintf("%lld:%lld ", iter->first, iter->second); }
	eprintf("\n");
}
void _debug_out_(map<char, ll> x) {
	auto begin = x.begin(), end = x.end();
	for (auto iter = begin; iter != end; iter++) { eprintf("%c:%lld ", iter->first, iter->second); }
	eprintf("\n");
}
#define debug_out(...) _debug_out_(__VA_ARGS__)
#else
#define debug_out(...) 42
#endif
//--debug--

int main() {
	ll N;
	cin >> N;
	vector<ll> A(N + 1);
	rep(i, N + 1) {
		cin >> A[i];
	}

	if (N == 0) {
		if (A[0] == 1) {
			cout << 1 << endl;
			return 0;
		}
		else {
			cout << -1 << endl;
			return 0;
		}
	}

	if (N > 0 && A[0] > 0) {
		cout << -1 << endl;
		return 0;
	}

	//その深さでの葉以外のノードの最大値
	vll notLeafMax(N + 1, 0);
	notLeafMax[0] = 1;
	reps(i, 1, N) {
		notLeafMax[i] = min(notLeafMax[i - 1] * 2ll - A[i], INF);
		if (notLeafMax[i] < 0) { //葉の要求しすぎでNG
			cout << -1 << endl;
			return 0;
		}
	}
	debug_out("notLeafMax:");
	debug_out(notLeafMax);

	//下から創る
	ll result = A[N];
	ll prevNode = A[N];
	deps(i, N - 1, 0) {
		//上からのキャップを、ノード全まとめ＋葉が上回ることある？ないはず
		ll a = prevNode + A[i];
		ll b = notLeafMax[i] + A[i];
		ll node = min(a, b);
		//debug_out("from prevNode:");
		//debug_out(a);
		//debug_out("from notLeafMax:");
		//debug_out(b);
		//debug_out("result node:");
		//debug_out(node);
		if (node <= 0) { //念のため
			cout << -1 << endl;
			return 0;
		}

		result += node;
		prevNode = node;
	}

	debug_out("result:");
	debug_out(result);

	if (result <= 0) {
		result = -1;
	}

	cout << result << endl;

	return 0;
}





//↓解説AC

//#include <iostream>
//#include <string>
//#include <vector>
//#include <algorithm>
//#include <cstring>
//#include <cmath>
//#include <limits>
//#include <stack>
//#include <queue>
//#include <map>
//#include <set>
//#include <functional>
//#include <iomanip>
//
//using namespace std;
//
//#ifdef LOCAL
//#define eprintf(...) fprintf(stderr, __VA_ARGS__)
//#else
//#define eprintf(...) 42
//#endif
//using ll = long long;
//using vll = std::vector<ll>;
//using vvll = std::vector<vll>;
//using vvvll = std::vector<vvll>;
//#define reps(i, S, E) for (ll i = (S); i <= (E); i++)
//#define rep(i, N) reps(i, 0, N-1)
//#define deps(i, E, S) for (ll i = (E); i >= (S); i--)
//#define dep(i, N) deps(i, N-1, 0)
////const ll INF = 1LL << 60;
////const int INF_INT = 1 << 30;
//const ll INF = 1000000000ll;
//const int INF_INT = 400000000;
//
//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; }
//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;
//	}
//};
//using vmll = std::vector<mll>;
//using vvmll = std::vector<vmll>;
//using vvvmll = std::vector<vvmll>;
//using vvvvmll = std::vector<vvvmll>;
//
//struct Fast {
//	Fast() {
//		cin.tie(0);
//		ios::sync_with_stdio(false);
//		cout << fixed << setprecision(std::numeric_limits<double>::max_digits10);
//	}
//} fast; //cin,cout高速化のおまじない＋桁数指定
//
//ll mll::MOD = (ll)(1e9 + 7);// 998244353ll;
//
////--debug--
//#ifdef LOCAL
//void _debug_out_(string s = "") {
//	if (s != "") { eprintf(s.c_str()); } else { eprintf("\n"); }
//}
//void _debug_out_(ll x, string s = "") {
//	if (s != "") { eprintf(s.c_str(), x); }	else { eprintf("%lld\n", x); }
//}
//void _debug_out_(mll x, string s = "") {
//	if (s != "") { eprintf(s.c_str(), x); }
//	else { eprintf("%lld\n", x.val); }
//}
//void _debug_out_(vvll x, string s = "") {
//	string t = s;
//	if (s == "") {
//		ll maxLen = 0;
//		rep(i, (ll)x.size()) rep(j, (ll)x[i].size()) {
//			ll n = x[i][j] / 10, len = 1;
//			while (n > 0) { len++; n /= 10; }
//			chmax(maxLen, len);
//		}
//		t = "%" + to_string(maxLen) + "lld ";
//	}
//	rep(i, (ll)x.size()) { rep(j, (ll)x[i].size()) { eprintf(t.c_str(), x[i][j]); } eprintf("\n"); }
//	eprintf("\n");
//}
//void _debug_out_(vll x, string s = "") {
//	rep(i, (ll)x.size()) { if (s != "") { eprintf(s.c_str(), x[i]); } else { eprintf("%lld ", x[i]); } }
//	eprintf("\n");
//}
//void _debug_out_(vector<bool> x) {
//	rep(i, (ll)x.size()) { eprintf("%s", x[i] ? "o" : "x");	}
//	eprintf("\n");
//}
//void _debug_out_(vector<vector<bool>> x) {
//	rep(i, (ll)x.size()) { rep(j, (ll)x[i].size()) { eprintf("%s", x[i][j] ? "o" : "x"); } eprintf("\n"); }
//	eprintf("\n");
//}
//void _debug_out_(map<ll, ll> x) {
//	auto begin = x.begin(), end = x.end();
//	for (auto iter = begin; iter != end; iter++) { eprintf("%lld:%lld ", iter->first, iter->second); }
//	eprintf("\n");
//}
//void _debug_out_(map<char, ll> x) {
//	auto begin = x.begin(), end = x.end();
//	for (auto iter = begin; iter != end; iter++) { eprintf("%c:%lld ", iter->first, iter->second); }
//	eprintf("\n");
//}
//#define debug_out(...) _debug_out_(__VA_ARGS__)
//#else
//#define debug_out(...) 42
//#endif
////--debug--
//
//int main() {
//	ll N;
//	cin >> N;
//	vector<ll> A(N + 1);
//	rep(i, N + 1) {
//		cin >> A[i];
//	}
//
//	if (N == 0) {
//		if (A[0] == 1) {
//			cout << 1 << endl;
//			return 0;
//		}
//		else {
//			cout << -1 << endl;
//			return 0;
//		}
//	}
//
//	if (N > 0 && A[0] > 0) {
//		cout << -1 << endl;
//		return 0;
//	}
//
//
//	vll totalLeaf(N + 2, 0);
//	totalLeaf[N] = A[N];
//	deps(i, N - 1, 0) {
//		totalLeaf[i] = totalLeaf[i + 1] + A[i];
//	}
//
//	ll notLeaf = 1;
//	ll result = 1;
//	reps(i, 1, N) {
//		ll temp = notLeaf * 2ll;
//		ll ans = min(temp, totalLeaf[i]);
//		result += ans;
//		notLeaf = ans - A[i];
//		if (notLeaf < 0) {
//			cout << -1 << endl;
//			return 0;
//		}
//	}
//
//	cout << result << endl;
//
//	return 0;
//}
//


Measured memory_bytes: 4764