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

typedef long long ll;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;
typedef vector<bool> vb;
typedef vector<vector<bool>> vvb;
typedef vector<vector<vector<bool>>> vvvb;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef vector<vpll> vvpll;
typedef vector<double> vd;
typedef vector<vd> vdd;

#define FOR(i,x,y) for(ll i=(ll)x; i<(ll)y; ++i)
#define REP(i,y) FOR(i, 0, y)
#define RFOR(i,x,y) for(ll i=(ll)x; i>=(ll)y; --i)
#define RREP(i,x) RFOR(i, x, 0)
#define ALL(a) a.begin(), a.end()
#define pb push_back
#define debug_print(x...) cerr << "line " << __LINE__ << " : "; debug_print_in(x);

template <typename First>
void debug_print_in(First first){
  cerr << first << endl;
  return;
}

template <typename First, typename... Rest>
void debug_print_in(First first, Rest... rest){
  cerr << first << " ";
  debug_print_in(rest...);
  return;
}

void IN(void){
  return;
}

template <typename First, typename... Rest>
void IN(First& first, Rest&... rest){
  cin >> first;
  IN(rest...);
  return;
}

template <typename First>
void OUT(First first){
  cout << first << endl;
  return;
}

template <typename First, typename... Rest>
void OUT(First first, Rest... rest){
  cout << first << " ";
  OUT(rest...);
  return;
}

template<class t, class u> void chmax(t&a,u b){if(a<b)a=b;};
template<class t, class u> void chmin(t&a,u b){if(a>b)a=b;};
int popcount(int t){return __builtin_popcount(t);} //GCC
int popcount(ll t){return __builtin_popcountll(t);} //GCC

template <typename T>
void vec_print(vector<T> VEC){
  REP(i, VEC.size()){
    cerr << VEC[i] << " ";
  }
  cerr << endl;
};

template <typename T>
void mat_print(vector<vector<T> > MAT){
  REP(i,MAT.size()){
    REP(j,MAT[i].size()){
      cerr << MAT[i][j] << " ";
    }
    cerr << endl;
  }
};

constexpr int INF = (1<<30);
constexpr ll INFLL = 1LL<<62;
constexpr long double EPS = 1e-12;
constexpr ll MOD = (ll)((1E+9)+7);

template<typename DATA>
class Segment_Tree{
  private:
    ll n;
    ll n_;

    using F = function<DATA(DATA,DATA)>;
    const F f; // query function
    const DATA init;

  public:
    vector<DATA> dat;
    vector<DATA> lazy1; //遅延評価 - http://tsutaj.hatenablog.com/entry/2017/03/30/224339
    vector<DATA> lazy2; //遅延評価 - http://tsutaj.hatenablog.com/entry/2017/03/30/224339
    vector<bool> lazy1_flag;
    vector<bool> lazy2_flag;
    Segment_Tree(ll n__, const F &f, DATA init) : f(f), init(init){
      // For simply, the number of component is set to be power of 2
      n = 1;
      n_ = n__;
      while(n < n_) n*=2;

      dat.resize(2*n-1, init);
      lazy1.resize(2*n-1, init);
      lazy2.resize(2*n-1, init);
      lazy1_flag.resize(2*n-1, false);
      lazy2_flag.resize(2*n-1, false);
      return;
    };

    ~Segment_Tree(void){
      return;
    };

    void eval1(ll k, ll l, ll r){
      if(!lazy1_flag[k]) return;
      if(l>=r) return;
      lazy1_flag[k] = false;

      dat[k] = lazy1[k];

      if(r-l>1){
        lazy1[2*k+1] = lazy1[k];
        lazy1[2*k+2] = lazy1[k];
        lazy1_flag[2*k+1] = true;
        lazy1_flag[2*k+2] = true;
      }

      lazy1[k] = init;
    }

    DATA eval2_2(DATA tmp, ll n){
      DATA ans = tmp;
      n--;
      while(n>0){
        if(n&1) ans = f(ans, tmp);
        tmp = f(tmp, tmp);
        n>>=1;
      }
      return ans;
    }

    void eval2(ll k, ll l, ll r){
      if(!lazy2_flag[k]) return;
      if(l>=r) return;
      lazy2_flag[k] = false;

      dat[k] = f(dat[k], eval2_2(lazy2[k], r-l));

      if(r-l>1){
        lazy2[2*k+1] = f(lazy2[2*k+1], lazy2[k]);
        lazy2[2*k+2] = f(lazy2[2*k+2], lazy2[k]);
        lazy2_flag[2*k+1] = true;
        lazy2_flag[2*k+2] = true;
      }

      lazy2[k] = init;
    }

    //k-th value is changed to a
    void update1(ll k, DATA a){
      k += n-1;
      dat[k] = a;
      while(k>0){
        k = (k-1)/2;
        dat[k] = f(dat[k*2+1], dat[k*2+2]);
      }
      return;
    };

    //k-th value is operated of a
    void update2(ll k, DATA a){
      k += n-1;
      dat[k] = f(dat[k], a);
      while(k>0){
        k = (k-1)/2;
        dat[k] = f(dat[k*2+1], dat[k*2+2]);
      }
      return;
    };

    //[a,b)の範囲の値をxに変える
    void rangeupdate1(ll a, ll b, DATA x, ll k, ll l, ll r){

      eval1(k,l,r);

      if(r<=a || b<=l) return;

      if(a<=l && r<=b){
        lazy1[k] = x;
        lazy1_flag[k] = true;
        //eval1(k,l,r);
      }else{
        rangeupdate1(a, b, x, 2*k+1, l, (l+r)/2);
        rangeupdate1(a, b, x, 2*k+2, (l+r)/2, r);
        dat[k] = f(dat[2*k+1], dat[2*k+2]);
      }
      return;
    }

    void rangeupdate1(ll a, ll b, DATA x){
      rangeupdate1(a,b,x,0,0,n);
    }

    //[a,b)の範囲に対してxを作用させる
    void rangeupdate2(ll a, ll b, DATA x, ll k, ll l, ll r){

      eval2(k,l,r);

      if(r<=a || b<=l) return;

      if(a<=l && r<=b){
        lazy2[k] = f(lazy2[k], x);
        lazy2_flag[k] = true;
        eval2(k,l,r);
      }else{
        rangeupdate2(a, b, x, 2*k+1, l, (l+r)/2);
        rangeupdate2(a, b, x, 2*k+2, (l+r)/2, r);
        dat[k] = f(dat[2*k+1], dat[2*k+2]);
      }

      return;
    }

    void rangeupdate2(ll a, ll b, DATA x){
      rangeupdate2(a,b,x,0,0,n);
    }

    // search the minimum of [a, b)
    // Other values are used for simply
    // From outside, call query(a, b, 0, 0, n)
    // k is the node index, 
    // temporary block is [l, r)
    DATA query(ll a, ll b, ll k, ll l, ll r){
      //if there are no overlap between [a, b) and [l, r)
      if(r<=a || b<=l) return init;

      eval1(k,l,r);
      eval2(k,l,r);

      //if [a, b) contains [l, r), return the value of this node
      if(a<=l && r<=b) return dat[k];

      DATA vl = query(a, b, k*2+1, l, (l+r)/2);
      DATA vr = query(a, b, k*2+2, (l+r)/2, r);
      return f(vl, vr);
    };

    // search the minimum of [a, b)
    DATA query(ll a, ll b){
      if(a<0 || n_<b || a==b){
        fprintf(stderr, "Segment_Tree Search Space Error\n");
        fprintf(stderr, "Desired:\n");
        fprintf(stderr, "%lld <= %lld < %lld <= %lld\n", 0, a, b, n_);
        exit(0);
      }
      return query(a, b, 0, 0, n);
    };
};

int main(){
  cin.tie(0); // cut the cin and cout (default, std::flush is performed after std::cin)
  ios::sync_with_stdio(false); // cut the iostream and stdio (DON'T endl; BUT "\n";)

  ll N;
  IN(N);
  vll Leaf(N+1, 0);
  REP(i,N+1){
    IN(Leaf[i]);
  }

  auto f = [](ll a, ll b){return a+b;};
  Segment_Tree<ll> Node(N+10, f, 0);

  if(N==0){
    if(Leaf[0]==1){
      OUT(1);
    }else{
      OUT(-1);
    }
    return 0;
  }

  if(Leaf[N]<=0 || Leaf[0]>0){
    OUT(-1);
    return 0;
  }

  ll target=0;
  REP(i,N+1){
    if(Leaf[i]==0) continue;
    ll add_leaf = Leaf[i];
    while(add_leaf>0){
      if(target==0){
        Node.rangeupdate2(target, i+1, 1);
        add_leaf--;
        target++;
      }else{
        if(target>i){
          OUT(-1);
          return 0;
        }
        ll additional = 1;
        if(target>0){
          additional = (Node.query(target-1, target)-Leaf[target-1])*2-Node.query(target,target+1);
        }

        //debug_print(i, add_leaf, target, additional);
        //debug_print(additional);
        if(additional>add_leaf){
          Node.rangeupdate2(target, i+1, add_leaf);
          add_leaf=0;
        }else{
          Node.rangeupdate2(target, i+1, additional);
          add_leaf-=additional;
          target++;
        }
      }
    }

    /*
    REP(i,N+1){
      debug_print(i, Node.query(i,i+1));
    }
    debug_print("target = ", target, "\n");
    */
  }

  OUT(Node.query(0, N+1));


  return 0;
}

Measured memory_bytes: 10344