#include <bits/stdc++.h>
using std::cin, std::cout, std::bitset, std::complex;
using std::vector, std::array, std::string, std::pair;
using std::priority_queue, std::set, std::map, std::unordered_map;
using std::sort, std::min, std::max, std::reverse;
using std::min_element, std::max_element, std::accumulate;

using vi = vector<int>; using vvi = vector<vi>; using vvvi = vector<vvi>;
using ll = long long int;
using vll = vector<ll>; using vvll = vector<vll>; using vvvll = vector<vvll>;
using vd = vector<double>; using vvd = vector<vd>; using vvvd = vector<vvd>;
using P = pair<int, int>;
using Pll = pair<ll, ll>;
using cdouble = complex<double>;

const double eps = 1e-7;
#define Loop(i, n) for(int i = 0; i < int(n); i++)
#define Loopll(i, n) for(ll i = 0; i < ll(n); i++)
#define Loop1(i, n) for(int i = 1; i <= int(n); i++)
#define Loopll1(i, n) for(ll i = 1; i <= ll(n); i++)
#define Loopr(i, n) for(int i = int(n) - 1; i >= 0; i--)
#define Looprll(i, n) for(ll i = ll(n) - 1; i >= 0; i--)
#define Loopr1(i, n) for(int i = int(n); i >= 1; i--)
#define Looprll1(i, n) for(ll i = ll(n); i >= 1; i--)
#define Foreach(buf, container) for(const auto &buf : container)
#define Foreachr(buf, container)  for(const auto &buf : reversed(container))
#define Loopdiag(i, j, h, w, sum) for(int i = ((sum) >= (h) ? (h) - 1 : (sum)), j = (sum) - i; i >= 0 && j < (w); i--, j++)
#define Loopdiagr(i, j, h, w, sum) for(int j = ((sum) >= (w) ? (w) - 1 : (sum)), i = (sum) - j; j >= 0 && i < (h); j--, i++)
#define Loopdiagsym(i, j, h, w, gap) for (int i = ((gap) >= 0 ? (gap) : 0), j = i - (gap); i < (h) && j < (w); i++, j++)
#define Loopdiagsymr(i, j, h, w, gap) for (int i = ((gap) > (h) - (w) - 1 ? (h) - 1 : (w) - 1 + (gap)), j = i - (gap); i >= 0 && j >= 0; i--, j--)
#define Loopitr(itr, container) for(auto itr = container.begin(); itr != container.end(); itr++)
#define quickio() std::ios::sync_with_stdio(false); std::cin.tie(0);
#define bitmanip(m,val) static_cast<bitset<(int)m>>(val)
#define Comp(type_t) bool operator<(const type_t &another) const
#define fst first
#define snd second
#define INF INFINITY
bool feq(double x, double y) { return abs(x - y) <= eps; }
bool inrange(ll x, ll t) { return x >= 0 && x < t; }
bool inrange(vll xs, ll t) { Foreach(x, xs) if (!(x >= 0 && x < t)) return false; return true; }
int ceillog2(ll x) { int m = int(log2(x)); return m + ((1LL << m) < x ? 1 : 0); }
int floorlog2(ll x) { int m = int(log2(x)); return m - ((1LL << m) > x ? 1 : 0); }
const string endl = "\n";
template<class T> T reversed(T container) { reverse(container.begin(), container.end()); return container; }
template<class T> void printv(const vector<T> &v) { for (const T &x : v) cout << x << " "; cout << endl; }
template<class T> void printmx(const vector<vector<T>> &mx) { for (const vector<T> &v : mx) printv(v); }
ll rndf(double x) { return (ll)(x + (x >= 0 ? 0.5 : -0.5)); }
ll floorsqrt(ll x) { ll m = (ll)sqrt((double)x); return m + (m*  m <= x ? 0 : -1); }
ll ceilsqrt(ll x) { ll m = (ll)sqrt((double)x); return m + (x <= m * m ? 0 : 1); }
ll rnddiv(ll a, ll b) { return (a / b + (a % b * 2 >= b ? 1 : 0)); }
ll ceildiv(ll a, ll b) { return (a / b + (a % b == 0 ? 0 : 1)); }
ll gcd(ll m, ll n) { if (n == 0) return m; else return gcd(n, m % n); }
ll lcm(ll m, ll n) { return ll(m) * ll(n) / gcd(m, n); }

//========================================================================//

class SegTreeSum {
	using val_t = ll;
private:
	struct segval_t {
		bool enable;
		val_t upd, add, sum;
	};
	int n, N; // n is the original size, while N is the extended size
	int base;
	vector<segval_t> nodes;
	vi idl, idr, cover_size;
	void merge(int id) {
		nodes[id].sum = nodes[idl[id]].sum + nodes[idl[id]].add * cover_size[idl[id]]
			+ nodes[idr[id]].sum + nodes[idr[id]].add * cover_size[idr[id]];
	}
	void lazy(int id) {
		if (id >= base) return;
		if (nodes[id].enable) {
			val_t upd = nodes[id].upd + nodes[id].add;
			nodes[idl[id]] = { true, upd, 0, upd * cover_size[idl[id]] };
			nodes[idr[id]] = { true, upd, 0, upd * cover_size[idr[id]] };
			nodes[id] = { false, 0, 0, upd * cover_size[id] };
		}
		else {
			nodes[idl[id]].add += nodes[id].add;
			nodes[idr[id]].add += nodes[id].add;
			nodes[id].add = 0;
			merge(id);
		}
	}
	enum change_t {
		UPD, ADD
	};
	void change_rec(int s, int t, int l, int r, int id, val_t x, change_t op) {
		if (s == l && t == r) {
			if (op == UPD) nodes[id] = { true, x, 0, x * cover_size[id] };
			if (op == ADD) nodes[id].add += x;
		}
		else {
			lazy(id);
			int m = (l + r) >> 1;
			if (s < m && m < t) {
				change_rec(s, m, l, m, idl[id], x, op);
				change_rec(m, t, m, r, idr[id], x, op);
			}
			else if (s < m) {
				change_rec(s, t, l, m, idl[id], x, op);
			}
			else if (m < t) {
				change_rec(s, t, m, r, idr[id], x, op);
			}
			merge(id);
		}
	}
	val_t solve_rec(int s, int t, int l, int r, int id) {
		val_t v = 0;
		if (s == l && t == r) {
			v = nodes[id].sum;
		}
		else {
			lazy(id);
			int m = (l + r) >> 1;
			if (s < m && m < t) {
				val_t v0 = solve_rec(s, m, l, m, idl[id]);
				val_t v1 = solve_rec(m, t, m, r, idr[id]);
				v = v0 + v1;
			}
			else if (s < m) {
				v = solve_rec(s, t, l, m, idl[id]);
			}
			else if (m < t) {
				v = solve_rec(s, t, m, r, idr[id]);
			}
		}
		v += nodes[id].add * (t - s);
		return v;
	}
	void common_init() {
		idl.resize(base + N, -1);
		idr.resize(base + N, -1);
		Loop(i, base) {
			idl[i] = (i << 1) + 1;
			idr[i] = (i << 1) + 2;
		}
		cover_size.resize(base + N);
		Loop(i, n) {
			cover_size[base + i] = 1;
		}
		Loopr(i, base) {
			cover_size[i] = cover_size[idl[i]] + cover_size[idr[i]];
		}
	}
public:
	SegTreeSum(int n, val_t init = 0) {
		this->n = n;
		N = 1 << ceillog2(n);
		base = N - 1;
		nodes = vector<segval_t>(base + N, { false, 0, 0, 0 });
		common_init();
		Loop(i, n) {
			nodes[base + i] = { true, init, 0, init };
		}
		Loopr(i, base) {
			merge(i);
		}
	}
	SegTreeSum(const vector<val_t> &a) {
		this->n = int(a.size());
		N = 1 << ceillog2(n);
		base = N - 1;
		nodes = vector<segval_t>(base + N, { false, 0, 0, 0 });
		common_init();
		Loop(i, n) {
			nodes[base + i] = { true, a[i], 0, a[i] };
		}
		Loopr(i, base) {
			merge(i);
		}
	}
	void upd(int s, int t, val_t x) {
		if (s >= t) return;
		change_rec(s, t, 0, N, 0, x, UPD);
	}
	void add(int s, int t, val_t x) {
		if (s >= t) return;
		change_rec(s, t, 0, N, 0, x, ADD);
	}
	val_t sumof(int s, int t) {
		if (s >= t) return 0;
		return solve_rec(s, t, 0, N, 0);
	}
};

int main() {
	quickio();
	int n; cin >> n;
	vll a(n + 1); Loop(i, n + 1) cin >> a[i];
	SegTreeSum *sts = new SegTreeSum(n + 3, 0); // only for branches
	int last = 0;
	Loop(i, n + 1) {
		if (i > 0) {
			ll buf = a[i];
			while (buf > 0) {
				ll leaf_cnt = a[last];
				ll branch_cap = (last > 0 ? (sts->sumof(last - 1, last) << 1) : 1);
				ll branch_cnt = sts->sumof(last, last + 1);
				if (last < i) {
					ll rem_cnt = branch_cap - (leaf_cnt + branch_cnt);
					if (rem_cnt == 0) {
						last++;
					}
					else {
						ll d_cnt = min(rem_cnt, buf);
						buf -= d_cnt;
						sts->add(last, i, d_cnt);
					}
				}
				else {
					if (branch_cap < leaf_cnt) {
						cout << -1 << endl;
						return 0;
					}
					else {
						buf = 0;
					}
				}
			}
		}
		else {
			if (a[i] == 1) {
				if (n == 0) {
					cout << 1 << endl;
					return 0;
				}
				else{
					cout << -1 << endl;
					return 0;
				}
			}
			else if (a[i] >= 2) {
				cout << -1 << endl;
				return 0;
			}
		}
	}
	cout << sts->sumof(0, n + 2) + accumulate(a.begin(), a.end(), 0LL) << endl;
	delete(sts);
}

Measured memory_bytes: 15568