#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <cassert>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <random>
#include <iomanip>
#include <unordered_set>
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int,int> > vpii;
typedef vector<pair<ll,ll> > vpll;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;
typedef vector<vc> vvc;
typedef vector<vs> vvs;
typedef vector<vll> vvll;
typedef map<int,int> mii;
typedef set<int> si;
#define rep(i,n) for(ll i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define arep(i,v) for(auto i : v)
#define irep(it, stl) for(auto it = stl.begin(); it != stl.end(); it++)
#define drep(i,n) for(int i = (n) - 1; i >= 0; --i)
#define fin(ans) cout << (ans) << '\n'
#define STLL(s) strtoll(s.c_str(), NULL, 10)
#define mp(p,q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define Rort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
#define fi first
#define se second
constexpr int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
constexpr int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
template <class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;} return 0;}
template <class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;} return 0;}
template <class T> inline void dump(T &v){irep(i, v){cout<<*i<<((i==--v.end())?'\n':' ');}}
inline string getline(){string s; getline(cin,s); return s;}
inline void yn(const bool b){b?fin("yes"):fin("no");}
inline void Yn(const bool b){b?fin("Yes"):fin("No");}
inline void YN(const bool b){b?fin("YES"):fin("NO");}
struct io{io(){ios::sync_with_stdio(false);cin.tie(0);}};
const int INF = INT_MAX;
constexpr ll LLINF = 1LL<<60;
constexpr ll MOD = 1000000007;
constexpr ld EPS = 1e-11;

int main() {

    int n;
    cin>>n;
    n++;
    vll a(n);
    rep(i,n)cin>>a[i];
    vll M(n);
    M[n-1]=0;
    for(int i=n-2;i>=0;i--){
        M[i]=a[i+1]+M[i+1];
    }
    if(a[0]!=1&&n==1){
        fin(-1);
        return 0;
    }
    if(n>1&&a[0]>0){
        fin(-1);
        return 0;
    }
    // dump(M);
    ll ans=0;
    ll lef=1;
    int i;
    for(i=0;i<n-1;i++){
        // cout<<i<<" "<<lef<<endl;
        ans+=min(lef,M[i])+a[i];
        lef=lef*2-a[i+1];
        if(lef<0){
            fin(-1);
            return 0;
        }
        if(lef>M[i+1])break;
    }
    for(++i;i<n-1;i++){
        ans+=M[i]+a[i];
    }
    ans+=a[n-1];
    fin(ans);
}






// int main() {

//     int h,m,h2,m2,k;
//     cin>>h>>m>>h2>>m2>>k;
//     int A,B;
//     A=h*60+m;
//     B=h2*60+m2;
//     B-=k;
//     cout<<A<<" "<<B<<endl;
//     fin(max(0,B-A));
// }



// int main() {

//     string s;
//     cin>>s;
//     rep(i,s.size())if(s[i]=='?')s[i]='D';
//     fin(s);
// }


Measured memory_bytes: 4844