#include <bits/stdc++.h>
#ifdef LOCAL_DEBUG
#define DEBUG 1
#define CERR if(DEBUG) cerr
#define MACRO(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,NAME,...) NAME
#define pr(...) CERR<<MACRO(__VA_ARGS__,pr10,pr9,pr8,pr7,pr6,pr5,pr4,pr3,pr2,pr1)(__VA_ARGS__)<<endl
#define pr1(a) (#a)<<"="<<(a)<<" "
#define pr2(a,b) pr1(a)<<pr1(b)
#define pr3(a,b,c) pr1(a)<<pr2(b,c)
#define pr4(a,b,c,d) pr1(a)<<pr3(b,c,d)
#define pr5(a,b,c,d,e) pr1(a)<<pr4(b,c,d,e)
#define pr6(a,b,c,d,e,f) pr1(a)<<pr5(b,c,d,e,f)
#define pr7(a,b,c,d,e,f,g) pr1(a)<<pr6(b,c,d,e,f,g)
#define pr8(a,b,c,d,e,f,g,h) pr1(a)<<pr7(b,c,d,e,f,g,h)
#define pr9(a,b,c,d,e,f,g,h,i) pr1(a)<<pr8(b,c,d,e,f,g,h,i)
#define pr10(a,b,c,d,e,f,g,h,i,j) pr1(a)<<pr9(b,c,d,e,f,g,h,i,j)
#define prArr(a){CERR<<(#a)<<"={";int i=0;for(auto t:(a))CERR<<(i++?", ":"")<<t;CERR<<"}"<<endl;}
#else
#define DEBUG 0
#define pr(...)
#define prArr(a)
#endif
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL<<60)+1e9; // ~ 1.15 * 1e18
const Int mod = (1e9)+7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int,Int>;
template<class T> T Max(T &a,T b){return a=max(a,b);}
template<class T> T Min(T &a,T b){return a=min(a,b);}
template<class T1, class T2> ostream& operator<<(ostream& o,pair<T1,T2> p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T1, class T2, class T3> ostream& operator<<(ostream& o,tuple<T1,T2,T3> t){return o<<"("<<get<0>(t)<<","<<get<1>(t)<<","<<get<2>(t)<<")";}
template<class T1, class T2> istream& operator>>(istream& i,pair<T1,T2> &p){return i>>p.first>>p.second;}
template<class T> ostream& operator<<(ostream& o,vector<T> a){Int i=0;for(T t:a)o<<(i++?" ":"")<<t;return o;}
template<class T> istream& operator>>(istream& i,vector<T> &a){for(T &t:a)i>>t;return i;}
//INSERT ABOVE HERE

template<typename D = long long, Int useAssert = 1>
class BIT{
public:
  Int n;
  vector<D> bit0, bit1;
  BIT(){n = -1;}
  BIT(Int n_){
    n = 1;
    while(n < n_) n *= 2;
    bit0.resize(n+1, 0);
    bit1.resize(n+1, 0);
  }

  D sum(vector<D> &b, Int i){
    D s = 0;
    while(i > 0){
      s = (s + b[i]);// % mod;
      i -= i & -i;
    }
    return s;
  }

  void add(vector<D> &b, Int i, D v){
    // v = (mod + v % mod) % mod;
    while(i <= n){
      b[i] = (b[i] + v);//% mod;
      i += i & -i;
    }
  }

  //[a,b)にxを加算 0オリジン
  void add (Int a, Int b, D x){
    // x = (mod + x % mod) % mod;
    if(a == b) return;
    a++;
    if(useAssert) assert(a <= b && 1 <= a && b <= n);
    add(bit0, a, -x*(a-1));
    add(bit1, a, x);
    add(bit0, b+1, x*b);
    add(bit1, b+1, -x);
  }

  //[a,b)のsumを得る 0オリジン
  D sum(Int a, Int b){
    if(a == b) return 0;
    a++;
    if(useAssert) assert(a <= b && 1 <= a && b <= n);
    D s = sum(bit0, b);
    D t = sum(bit1, b) * b;
    D u = sum(bit0, a-1);
    D v = sum(bit1, a-1) * (a-1);
    return s + t - u - v;
    //return ((s + t) + (mod - u%mod) + (mod - v%mod)) % mod;
  }
};


signed main(){
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);

  Int N;
  cin>>N;
  vector<Int> A(N + 1);
  cin>>A;

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

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

  BIT <Int> bit(N + 10);
  bit.add(0, 1, 2);
  Int idx = 0;
  Int ans = 1;
  for(Int i=1;i<=N;i++){
    while(A[i]){
      while(idx <= N+5 && bit.sum(idx, idx + 1) == 0)idx++;
      if(idx >= i){
        cout<<-1<<endl;
        return 0;
      }
      Int num = min(bit.sum(idx, idx + 1), A[i]);
      bit.add(idx, idx + 1, - num);
      A[i] -= num;
      ans += (i - idx) * num;
      bit.add(idx + 1, i, num);

    }
  }
  cout<<ans<<endl;

  return 0;
}


Measured memory_bytes: 6100