#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>  
#include <ext/pb_ds/tree_policy.hpp>   
using namespace std;
using namespace __gnu_pbds;  
typedef tree<int, null_type, less<int>, rb_tree_tag,  tree_order_statistics_node_update>  ordered_set; 
#define num 1000000007
#define num2 998244353
#define REP(i,a,n) for(ll i=a;i<n;i++)
#define REPd(i,a,n) for(ll i=a; i>=n; i--)
#define vll vector<long long>
#define pll pair<long long, long long>
#define vpll vector< pair<long long, long long>>
#define pb push_back
#define pob pop_back
#define f first
#define s second
#define fix(f,n) std::fixed<<std::setprecision(n)<<f
#define all(x) x.begin(), x.end()
#define M_PI 3.14159265358979323846
#define epsilon (double)(0.00000000001)
#define watch(x) cerr << (#x) << " is " << (x) << endl 
typedef long long ll;

// DEBUG FUNCTIONS FOR TEST 
#ifndef ONLINE_JUDGE

#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }

void err(istream_iterator<string> it) {}
template<typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
	cerr << *it << " = " << a << endl;
	err(++it, args...);
}

template<typename T>
void __p(T a) {
  cout<<a;
}
template<typename T, typename F>
void __p(pair<T, F> a) {
  cout<<"{";
  __p(a.first);
  cout<<",";
  __p(a.second);
  cout<<"}";
}
template<typename T>
void __p(std::vector<T> a) {
  cout<<"{";
  for(auto it=a.begin(); it<a.end(); it++)
    __p(*it),cout<<",}"[it+1==a.end()];
}
template<typename T, typename ...Arg>
void __p(T a1, Arg ...a) {
  __p(a1);
  __p(a...);
}
template<typename Arg1>
void __f(const char *name, Arg1 &&arg1) {
  cout<<name<<" : ";
  __p(arg1);
  cout<<endl;
}
template<typename Arg1, typename ... Args>
void __f(const char *names, Arg1 &&arg1, Args &&... args) {
  int bracket=0,i=0;
  for(;; i++)
    if(names[i]==','&&bracket==0)
      break;
    else if(names[i]=='(')
      bracket++;
    else if(names[i]==')')
      bracket--;
  const char *comma=names+i;
  cout.write(names,comma-names)<<" : ";
  __p(arg1);
  cout<<" | ";
  __f(comma+1,args...);
}
#define trace(...) cout<<"Line:"<<__LINE__<<" ", __f(#__VA_ARGS__, __VA_ARGS__)
#else
#define trace(...)
#define error(...)
#define endl '\n'
#endif

// DEBUG FUNCTIONS END 
ll mod(ll a, ll b)
{
	if(a%b<0)
	{
		return a%b + b;
	}
	return a%b;
}
 
 
ll mod_exp(ll a, ll b, ll c)
{
	ll res=1;
	a=a%c;
	while(b>0)
	{
		if(b%2==1)
		{
			res=(res*a)%c;
		}
		b/=2;
		a=(a*a)%c;
	}
	return res;
}
ll mymod(ll a,ll b)
{
    return ((a%b)+b)%b;
}
bool prime[200001];
void SieveOfEratosthenes() 
{ 
    memset(prime, true, sizeof(prime)); 
    prime[1]=false;
    for (int p=2; p*p<=200000; p++) 
    { 
        if (prime[p] == true) 
        { 
            for (int i=p*p; i<=200000; i += p) 
                prime[i] = false; 
        } 
    } 
    return;
}
ll powe[200005];
void power()
{
    powe[0]=1;
    REP(i,1,200005)
    {
        powe[i]=mymod(2*powe[i-1],num);
    }
}
ll gcdExtended(ll,ll,ll *,ll *);
ll modInverse(ll a, ll m) 
{ 
    ll x, y; 
    ll g = gcdExtended(a, m, &x, &y); 
        ll res = (x%m + m) % m;
        return res;
} 
ll gcdExtended(ll a, ll b, ll *x, ll *y) 
{ 
    if (a == 0) 
    { 
        *x = 0, *y = 1; 
        return b; 
    } 
    ll x1, y1; 
    ll gcd = gcdExtended(b%a, a, &x1, &y1); 
    *x = y1 - (b/a) * x1; 
    *y = x1; 
    return gcd; 
} 
ll facto[200005];
void f()
{
	facto[0]=1;
	REP(i,1,200005)
	{
		facto[i]=mymod((i*facto[i-1]),num);
	}
}
struct node
{
    ll degree;
    bool vis;
    vll adj;
};
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    ll n;
    cin>>n;
	
	ll leaf[n+1];
    REP(i,0,n+1)
	{
		cin>>leaf[i];
	}
	vll suf(n+2, 0);
	REPd(i,n,0)
	{
		suf[i] = suf[i+1] + leaf[i];
	}
	ll maxnodes[n+1];
	ll nonleaf[n+1];
	ll ans = 1;
	maxnodes[0] = 1;
	nonleaf[0] = 1 - leaf[0];
	REP(i,1,n+1)
	{
		maxnodes[i] = min(2*nonleaf[i-1], suf[i+1]+leaf[i]);
		nonleaf[i] = maxnodes[i] - leaf[i];
		if(maxnodes[i]<leaf[i])
		{
			cout<<"-1\n";
			exit(0);
		}
		ans+=maxnodes[i];
	}
	if(n==0 && leaf[0]>1)
	{
		cout<<"-1\n";
		exit(0);
	}
	cout<<ans<<"\n";
	return 0;
}


Measured memory_bytes: 6372