#pragma target("avx")
#pragma optimize("O3")
#include<bits/stdc++.h>
using ll = int_fast64_t;
using P = std::pair<ll,ll>;
using PP = std::pair<ll,P>;
using V = std::vector<ll>;
template<typename T> using pq = std::priority_queue<T>;
template<typename T> using rpq = std::priority_queue<T, std::vector<T>, std::greater<T>>;

#define REP(i,b,e) for(ll i=b; i<e; i++)
#define ALL(vec) vec.begin(),vec.end()
#define PRINT(vec) printf("[ ");for(auto &i:vec)printf("%ld ",i);puts("]");
#define SCAN(vec) for(auto &i:vec)scanf("%ld",&i)
#define fi first
#define se second

const int MOD = 1e9+7;
const ll INF = 1e18;
int dx[] = {0, 1, 0, -1, 1, 1, -1, -1}, dy[] = {1, 0, -1, 0, 1, -1, -1, 1};

using namespace std;
template< typename Monoid, typename OperatorMonoid = Monoid >
struct LazySegmentTree {
  using F = function< Monoid(Monoid, Monoid) >;
  using G = function< Monoid(Monoid, OperatorMonoid) >;
  using H = function< OperatorMonoid(OperatorMonoid, OperatorMonoid) >;

  int sz, height;
  vector< Monoid > data;
  vector< OperatorMonoid > lazy;
  const F f;
  const G g;
  const H h;
  const Monoid M1;
  const OperatorMonoid OM0;


  LazySegmentTree(int n, const F f, const G g, const H h,
                  const Monoid &M1, const OperatorMonoid OM0)
      : f(f), g(g), h(h), M1(M1), OM0(OM0) {
    sz = 1;
    height = 0;
    while(sz < n) sz <<= 1, height++;
    data.assign(2 * sz, M1);
    lazy.assign(2 * sz, OM0);
  }

  void set(int k, const Monoid &x) {
    data[k + sz] = x;
  }

  void build() {
    for(int k = sz - 1; k > 0; k--) {
      data[k] = f(data[2 * k + 0], data[2 * k + 1]);
    }
  }

  inline void propagate(int k) {
    if(lazy[k] != OM0) {
      lazy[2 * k + 0] = h(lazy[2 * k + 0], lazy[k]);
      lazy[2 * k + 1] = h(lazy[2 * k + 1], lazy[k]);
      data[k] = reflect(k);
      lazy[k] = OM0;
    }
  }

  inline Monoid reflect(int k) {
    return lazy[k] == OM0 ? data[k] : g(data[k], lazy[k]);
  }

  inline void recalc(int k) {
    while(k >>= 1) data[k] = f(reflect(2 * k + 0), reflect(2 * k + 1));
  }

  inline void thrust(int k) {
    for(int i = height; i > 0; i--) propagate(k >> i);
  }

  void update(int a, int b, const OperatorMonoid &x) {
    thrust(a += sz);
    thrust(b += sz - 1);
    for(int l = a, r = b + 1; l < r; l >>= 1, r >>= 1) {
      if(l & 1) lazy[l] = h(lazy[l], x), ++l;
      if(r & 1) --r, lazy[r] = h(lazy[r], x);
    }
    recalc(a);
    recalc(b);
  }

  Monoid query(int a, int b) {
    thrust(a += sz);
    thrust(b += sz - 1);
    Monoid L = M1, R = M1;
    for(int l = a, r = b + 1; l < r; l >>= 1, r >>= 1) {
      if(l & 1) L = f(L, reflect(l++));
      if(r & 1) R = f(reflect(--r), R);
    }
    return f(L, R);
  }

  Monoid operator[](const int &k) {
    return query(k, k + 1);
  }

  template< typename C >
  int find_subtree(int a, const C &check, Monoid &M, bool type) {
    while(a < sz) {
      propagate(a);
      Monoid nxt = type ? f(reflect(2 * a + type), M) : f(M, reflect(2 * a + type));
      if(check(nxt)) a = 2 * a + type;
      else M = nxt, a = 2 * a + 1 - type;
    }
    return a - sz;
  }

  template< typename C >
  int find_first(int a, const C &check) {
    Monoid L = M1;
    if(a <= 0) {
      if(check(f(L, reflect(1)))) return find_subtree(1, check, L, false);
      return -1;
    }
    thrust(a + sz);
    int b = sz;
    for(a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if(a & 1) {
        Monoid nxt = f(L, reflect(a));
        if(check(nxt)) return find_subtree(a, check, L, false);
        L = nxt;
        ++a;
      }
    }
    return -1;
  }


  template< typename C >
  int find_last(int b, const C &check) {
    Monoid R = M1;
    if(b >= sz) {
      if(check(f(reflect(1), R))) return find_subtree(1, check, R, true);
      return -1;
    }
    thrust(b + sz - 1);
    int a = sz;
    for(b += sz; a < b; a >>= 1, b >>= 1) {
      if(b & 1) {
        Monoid nxt = f(reflect(--b), R);
        if(check(nxt)) return find_subtree(b, check, R, true);
        R = nxt;
      }
    }
    return -1;
  }
};


int main(){
	int n;
	scanf("%d", &n);
	V a(n+1);
	SCAN(a);
	if(n==0){
		if(a[0]==1) puts("1");
		else puts("-1");
		return 0;
	}else if(a[0]!=0){
		puts("-1");
		return 0;
	}
	std::reverse(ALL(a));

	ll usd = 0;
	REP(i, 0, n) usd = (usd+a[i]+1)/2;
	if(usd!=1){
		puts("-1");
		return 0;
	}
	
	LazySegmentTree<ll,ll> tree(n+1,
		[](ll a, ll b){return a+b;},
		[](ll a, ll b){return a+b;},
		[](ll a, ll b){return a+b;},
		0, 0
	);

	tree.update(0, n, 1);
	ll ans = n+1, pos = 0;
	std::reverse(ALL(a));
	a[n]--;
	REP(i, 0, n+1) while(a[i]) {
		if(a[i]<=tree[pos]){
			ans += (i - pos) * a[i];
			tree.update(pos, pos+1, -a[i]);
			tree.update(pos+1, i, a[i]);
			a[i] = 0;
		}else{
			ans += (i - pos) * tree[pos];
			tree.update(pos+1, i, tree[pos]);
			a[i] -= tree[pos];
			tree.update(pos, pos+1, -tree[pos]);
		}
		while(tree[pos]==0) pos++;
	}
	printf("%ld\n", ans);
	return 0;
}


Measured memory_bytes: 8296