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

#ifdef DEBUG
    #include "debug.h"
    #define DEB
    #define dump(...)                                                              \
        DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                            \
                << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
                << endl                                                            \
                << "    ",                                                         \
            dump_func(__VA_ARGS__)
#else
    #define DEB if (false)
    #define dump(...)
#endif


// http://beet-aizu.hatenablog.com/entry/2018/04/08/145516
template<typename T>
vector<T> make_v(size_t a){return vector<T>(a);}

template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

template<typename T,typename V>
typename enable_if<is_class<T>::value==0>::type
fill_v(T &t,const V &v){t=v;}

template<typename T,typename V>
typename enable_if<is_class<T>::value!=0>::type
fill_v(T &t,const V &v){
  for(auto &e:t) fill_v(e,v);
}

//////////////////

#define int long long
 
#define rep(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)

#define YES puts("YES")
#define Yes puts("Yes")
#define NO  puts("NO")
#define No  puts("No")
#define ALL(v) (v).begin(), (v).end()
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

//*
#define mod 1000000007 /*/
#define mod 998244353 //*/

typedef pair<int, int> P;
#define INF (1LL<<60)

struct SegmentTree {
private:
    int n;
    vector<int> node;

public:
    SegmentTree(vector<int> v) {
        int sz = v.size();
        n = 1; while(n < sz) n *= 2;
        node.resize(2*n-1, -INF);
        for(int i=0; i<sz; i++) node[i+n-1] = v[i];
        for(int i=n-2; i>=0; i--) node[i] = max(node[2*i+1], node[2*i+2]);
    }
    void update(int x, int val) {
        x += (n - 1);
        node[x] = val;
        while(x > 0) {
            x = (x - 1) / 2;
            node[x] = max(node[2*x+1], node[2*x+2]);
        }
    }
    int getmax(int a, int b, int k=0, int l=0, int r=-1) {
        if(r < 0) r = n;
        if(r <= a || b <= l) return -INF;
        if(a <= l && r <= b) return node[k];
        int vl = getmax(a, b, 2*k+1, l, (l+r)/2);
        int vr = getmax(a, b, 2*k+2, (l+r)/2, r);
        return max(vl, vr);
    }
};

void solve(){
    int N;
    cin >> N;
    vector<int> A(N+1);
    rep(i,N+1) cin >> A[i];
    vector<int> sum(N+2, 0);
    rep(i,N+1) sum[i+1] = sum[i] + A[i];
    
    SegmentTree seg(A);


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

    if(A[0]!=0){ cout << -1 << endl; return;}
    
    int ans = 1;
    vector<int> amari(N+2,0);
    amari[0]=1;
    for(int i=1; i<N+1; i++){
        
        amari[i] = min(amari[i-1] * 2, sum[N+1] - sum[i]);
        dump(i,amari);
        ans += amari[i];

        amari[i]-=A[i];
        dump(i,amari);
        if(amari[i]<0){ cout << -1 << endl; return;}
        
    }
    cout << ans << endl;
    
}


signed main(){
    cout << fixed << setprecision(18);
    cerr << fixed << setprecision(18);
    solve();
}


Measured memory_bytes: 7772