#include <bits/stdc++.h>
using namespace std;
//using Graph = vector<vector<edge>>;
using dou =long double;
string yes="yes";
string Yes="Yes";
string YES="YES";
string no="no";
string No="No";
string NO="NO";

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; }

#define rep(i, n)         for(int i = 0; i < (int)(n); i++)
#define brep(n)           for(int bit=0;bit<(1<<n);bit++)
#define erep(i,container) for (auto i : container)
#define irep(i, n)        for(int i = n-1; i >= (int)0; i--)
#define rrep(i,m,n) for(ll i = m; i < (ll)(n); i++)
#define reprep(i,j,h,w) rep(i,h)rep(j,w)
#define all(x) (x).begin(),(x).end()
#define VEC(type,name,n) std::vector<type> name(n);rep(i,n)std::cin >> name[i];
#define pb push_back
#define pf push_front
#define query int qq;std::cin >> qq;rep(qqq,qq)
#define lb lower_bound
#define ub upper_bound
#define fi first
#define se second
#define itn int
#define mp make_pair
#define sum(a) accumulate(all(a),0ll)
#define keta fixed<<setprecision
#define vvector(name,typ,m,n,a)vector<vector<typ> > name(m,vector<typ> (n,a))
//#define vvector(name,typ,m,n)vector<vector<typ> > name(m,vector<typ> (n))
#define vvvector(name,t,l,m,n,a) vector<vector<vector<t> > > name(l, vector<vector<t> >(m, vector<t>(n,a)));
#define vvvvector(name,t,k,l,m,n,a) vector<vector<vector<vector<t> > > > name(k,vector<vector<vector<t> > >(l, vector<vector<t> >(m, vector<t>(n,a)) ));
#define case std::cout <<"Case #" <<qqq+1<<": "
#define res resize
#define as assign
#define ffor for(;;)
#define ppri(a,b) std::cout << a<<" "<<b << std::endl
#define pppri(a,b,c) std::cout << a<<" "<<b <<" "<< c<<std::endl
#define aall(x,n) (x).begin(),(x).begin()+(n)
#define ssum(a) accumulate(a,0ll) 

typedef long long ll;
typedef pair<int,int> P;
typedef pair<ll,ll> PL;
//typedef long long T;
ll ceil(ll a,ll b){
    return ((a+b-1)/b);
}
const int INF = 2000000000;
//const ll INF64 = 9223372036854775807ll;
const ll INF64 = 9223372036854775ll;
const ll MOD = 1000000007ll;
const dou pi=3.141592653589793;
//mapは、if関数などで覗いただけで構成されるので注意しよう！
//std::vector<vector<int>> g;
int main(){
    int n;
    std::cin >> n;
    VEC(ll,a,n+1);
    if(n==0&&a[0]==0){
        std::cout << -1 << std::endl;
        exit(0);
    }
    std::vector<ll> ia(a);
    reverse(all(ia));
    ll cn=0;
    std::vector<ll> ma(n+2);
    irep(i,n+1){
        cn+=a[i];
        ma[i]+=(cn);
        //std::cout << ma << std::endl;
    }
    //std::cout << ma << std::endl;
    std::vector<ll> me;
    me.pb(1);
    bool ddd=0;
    rrep(i,1,n+1){
        ll du=(me[i-1]-a[i-1])*2;
        if(du>=INF64)ddd=1;
        if(ddd)me.pb(INF64);
        else me.pb(du);
    }
    ll ans=0;
    bool han=1;
    /*
    rep(i,n+1){
        if(me[i]==0){
            std::cout << -1 << std::endl;
            exit(0);
        }
    }
    */
    /*
    erep(i,me){
        std::cout << i << std::endl;
    }
    */
    rep(i,n+1){
        if(a[i]>me[i]){
            std::cout << -1 << std::endl;
            exit(0);
        }
        if(me[i]<=ma[i])ans+=me[i];
        else ans+=ma[i];
    }
    if(ans==0)std::cout << 0 << std::endl;
    else std::cout << ans << std::endl;
    
}

Measured memory_bytes: 6808