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

#define int long long
#define vi vector<int>
#define ii pair<int,int>
#define fi first
#define sc second
#define stoi stoll
#define popcnt __builtin_popcount
#define getbit(x, k) ((x >> k) & 1)
#define all(x) (x).begin(),(x).end()
#define FOR(i,j,k) for(int i=j; i<k; i++)
#define look(a) cerr <<#a<<": "<<a<<endl;
#define look2(a,b) cerr <<#a<<": "<<a<<" | "<<#b<<": "<<b<< endl;

void solve()
{
	int n;
	cin >> n;
	int a[n+1],b[n+1],c[n+1];
	FOR(i,0,n+1) cin >> a[i];
	if (a[0] > 1){
		cout << -1;
		return;
	}
	c[n] = a[n];
	for (int i = n-1; i >= 0; i--){
		c[i] = c[i+1] + a[i];
	}
	b[0] = 1 - a[0];
	FOR(i,1,n+1){
		if (a[i] > b[i-1]*2){
			cout << -1;
			return;
		}
		b[i] = min(c[i+1],b[i-1]*2 - a[i]);
		//look2(c[i+1],b[i-1]*2 - a[i])
		if (a[i] + b[i] < b[i-1]){
			cout << -1;
			return;
		}
	}
	//FOR(i,0,n+1) look2(a[i],b[i]);
	int s = a[n];
	FOR(i,0,n) s += a[i] + b[i];
	cout << s;
}

signed main()
{
	cin.tie(0)->sync_with_stdio(0);
	freopen("in", "r", stdin);
	freopen("out", "w", stdout);
	int T = 1;
	// cin >> T;
	while (T--) {
		solve();
		cout << '\n';
	}
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
}