// Slower C++ solution:
#include <iostream>

#include <fstream>

#include <vector>

#include <algorithm>

#include <cmath>

#include <limits>

#include <queue>

#include <iomanip>

#include <set>

//#include <bits/stdc++.h>

template<typename T> bool chmax(T &a,T b){if(a<b){a=b;return true;}return false;}

template<typename T> bool chmin(T &a,T b){if(a>b){a=b;return true;}return false;}

using namespace std;

#define ALL(X) X.begin(),X.end()

using ll = long long int;

typedef vector<ll> vll;

typedef vector<vll> vvll;

typedef vector<vvll> vvvll;

typedef vector<bool> vb;

typedef vector<vb> vvb;

const int MOD=1000000007;

//const int MOD=998244353;

const int INTMAX=2147483647;

const ll LLMAX=9223372036854775807;





int main(){

    ios::sync_with_stdio(false);

    cin.tie(0);

    

    /*ll N;

    cin>>N;

    

    vll A(N+1);

    for(ll i=0;i<=N;i++)cin>>A[i];

    if(N==0){

        if(A[0]==1){

            cout<<1<<endl;

            return 0;

        }else {

            cout<<-1<<endl;

            return 0;

        }

    }

    if(A[0]!=0){

        cout<<-1<<endl;

        return 0;

    }

    

    

    vll B(N+1);

    for(ll i=0;i<=N;i++){

        if(i==0){

            B[0]=1;

        }else{

            B[i]=(B[i-1]-A[i-1])*2;

            if(B[i]<A[i]){

                cout<<-1<<endl;

                return 0;

            }

        }

    }

    

    vll C(N+1);

    for(ll i=N;i>=0;i--){

        if(i==N){

            C[i]=A[i];

        }else{

            C[i]=C[i+1]+A[i];

        }

    }

    

    ll ans=0;

    for(ll i=0;i<=N;i++){ans+=min(B[i],C[i]);

    }

    cout<<ans<<endl;

    

    return 0;

}



int main() {*/

    ll n,a[100005]={0},b[100005],c[100005],pv,ans=0;

      bool ok=false;

    cin >> n;

    for(ll i=0;i<=n;i++){

        cin >> a[i];

    }

    for(ll i=0;i<=n;i++){

        if(i==0){

            if(a[i]>1){

                cout << -1 << endl;

                return 0;

            }

            b[0]=1;

        } else {

            if(ok) b[i]=2*1e17;

            else b[i]=(b[i-1]-a[i-1])*2;

              if(b[i]>=1e17) ok=true;

            if(a[i]>b[i]){

                cout << -1 << endl;

                return 0;

            }

        }

    }

    b[n+1]=1;

    for(ll i=n;i>=0;i--){

        if(i==n){

            c[i]=a[i];

        } else {

            c[i]=c[i+1]+a[i];

        }

    }

    c[n+1]=0;

    for(ll i=0;i<=n+1;i++){

        if(b[i]>c[i]){

            pv=i;

            break;

        }

    }

    for(ll i=0;i<=n;i++){

        if(i<pv){

            ans+=b[i];

        } else {

            ans+=c[i];

        }

    }

    cout << ans << endl;

    return 0;

}


// Faster C++ solution for the same problem (measured speedup: 1.8196784389603156x):
#include <iostream>

#include <fstream>

#include <vector>

#include <algorithm>

#include <cmath>

#include <limits>

#include <queue>

#include <iomanip>

#include <set>

//#include <bits/stdc++.h>

template<typename T> bool chmax(T &a,T b){if(a<b){a=b;return true;}return false;}

template<typename T> bool chmin(T &a,T b){if(a>b){a=b;return true;}return false;}

using namespace std;

#define ALL(X) X.begin(),X.end()

using ll = long long int;

typedef vector<ll> vll;

typedef vector<vll> vvll;

typedef vector<vvll> vvvll;

typedef vector<bool> vb;

typedef vector<vb> vvb;

const int MOD=1000000007;

//const int MOD=998244353;

const int INTMAX=2147483647;

const ll LLMAX=9223372036854775807;





int main(){

    ios::sync_with_stdio(false);

    cin.tie(0);

    

    /*ll N;

     cin>>N;

     

     vll A(N+1);

     for(ll i=0;i<=N;i++)cin>>A[i];

     if(N==0){

     if(A[0]==1){

     cout<<1<<endl;

     return 0;

     }else {

     cout<<-1<<endl;

     return 0;

     }

     }

     if(A[0]!=0){

     cout<<-1<<endl;

     return 0;

     }

     

     

     vll B(N+1);

     for(ll i=0;i<=N;i++){

     if(i==0){

     B[0]=1;

     }else{

     B[i]=(B[i-1]-A[i-1])*2;

     if(B[i]<A[i]){

     cout<<-1<<endl;

     return 0;

     }

     }

     }

     

     vll C(N+1);

     for(ll i=N;i>=0;i--){

     if(i==N){

     C[i]=A[i];

     }else{

     C[i]=C[i+1]+A[i];

     }

     }

     

     ll ans=0;

     for(ll i=0;i<=N;i++){ans+=min(B[i],C[i]);

     }

     cout<<ans<<endl;

     

     return 0;

     }

     

     int main() {*/

    //ll n,a[100005]={0},b[100005],c[100005],pv,ans=0;

    ll n,pv;

    

    //vll a(100005),b(100005),c(100005);

    bool ok=false;

    cin >> n;

    vll a(n+2),b(n+2),c(n+2);

    for(ll i=0;i<=n;i++){

        cin >> a[i];

    }

    for(ll i=0;i<=n;i++){

        if(i==0){

            if(a[i]>1){

                cout << -1 << endl;

                return 0;

            }

            b[0]=1;

        } else {

            if(ok) b[i]=2*1e17;

            else b[i]=(b[i-1]-a[i-1])*2;

            if(b[i]>=1e17) ok=true;

            if(a[i]>b[i]){

                cout << -1 << endl;

                return 0;

            }

        }

    }

    b[n+1]=1;

    for(ll i=n;i>=0;i--){

        if(i==n){

            c[i]=a[i];

        } else {

            c[i]=c[i+1]+a[i];

        }

    }

    c[n+1]=0;

    for(ll i=0;i<=n+1;i++){

        if(b[i]>c[i]){

            pv=i;

            break;

        }

    }

    ll ans=0;

    for(ll i=0;i<=n;i++){

        if(i<pv){

            ans+=b[i];

        } else {

            ans+=c[i];

        }

    }

    cout << ans << endl;

    return 0;

}