// Marcin Knapik
#include<bits/stdc++.h>
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")

using namespace std;

// #include <ext/pb_ds/assoc_container.hpp> // order_of_key
// #include <ext/pb_ds/tree_policy.hpp> // find by order
// using namespace __gnu_pbds; 
// #define ordered_set tree< int , null_type,less< int >, rb_tree_tag,tree_order_statistics_node_update>  

// const ll MAX_LL = 0xFFFFFFFFFFFFFFFF;
// mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
#define losuj(a, b) uniform_int_distribution<ll>(a, b)(rng)

typedef long double             ld;
typedef long long               ll;
typedef pair<int, int>          ii;
typedef vector<int>             vi;
typedef vector<ll>              vll;
typedef vector<vll>             vvll;
typedef vector<ii>              vii;
typedef vector<vi>              vvi;
typedef vector<vii>             vvii;
#define pll pair<long long, long long>
#define ull unsigned long long

#define sz(a)                   (int)(a).size()
#define pb                      push_back
#define all(c)                  (c).begin(), (c).end()
#define rall(c)                 (c).rbegin(), (c).rend()
#define FOR(i, a)               for (int i = 0; i < (a); i++)
#define f                       first
#define s                       second

template <class T> inline bool setmin(T &a, T b){if (a > b)return a = b, 1;return 0;}
template <class T> inline bool setmax(T &a, T b){if (a < b)return a = b, 1;return 0;}

template<class T> inline T fast(T a,T b,T mod) {ll res = 1; while(b){if(b&1) res = (res*a)%mod;a = (a*a)%mod;b >>= 1;}return res;}
template<class T> istream &operator>>(istream &os, vector<T> &container){for (auto &u : container)os >> u;return os;}
template<class T> ostream &operator<<(ostream &os, const vector<T> &container){for (auto &u : container)os << u << " ";return os;}
template<class T> inline T gcd(T a, T b) { while (b)swap(a %= b, b); return a; }

const ll INF = 1e9 + 7;
const ll mod = 998244353;
const ll BIG_INF = 1e18 + 7;
const ll N = 1e6+7;
const ll T = 1<<17;
const ld inf = 1e18;
ll m, n, k, q;

ll tree[T * 2];
ll tab[N];

void add(int pocz, int kon, ll val){
    if(pocz > kon)
        return;
    pocz += T - 1;
    kon += T + 1;

    while(pocz / 2 != kon / 2){
        if(pocz % 2 == 0){
            tree[pocz + 1] += val;
            tree[pocz + 1] = min(tree[pocz + 1], BIG_INF);
        }
        if(kon % 2 == 1){
            tree[kon - 1] += val;
            tree[kon - 1] = min(tree[kon - 1], BIG_INF);
        }
        pocz /= 2;
        kon /= 2;
    }
}

ll ask(int poz){
    poz += T;

    ll ret = 0;

    while(poz){
        ret += tree[poz];
        ret = min(ret, BIG_INF);
        poz /= 2;
    }
    return ret;
}

signed main(){
    ios::sync_with_stdio(0);
    cin.tie(0);

    cin >> n;
    add(1, 1, 1);

    for(int i = 1; i <= n + 1; i++)
        cin >> tab[i];
    
    int wsk = 1;
    ll ans = 0;

    for(int i = 1; i <= n + 1; i++){
        while(wsk <= i and tab[i]){
            ll pom = ask(wsk);
            ll dist = (i - wsk + 1);

            if(pom >= tab[i]){
                ans += dist * tab[i];
                add(wsk, wsk, -tab[i]);
                add(wsk + 1, i, tab[i]);
                tab[i] = 0;
            }
            else{
                ans += dist * pom;
                add(wsk, wsk, -pom);
                add(wsk + 1, i, pom);
                wsk++;
                tab[i] -= pom;
            }
        }
        if(wsk > i and tab[i]){
            cout << -1 << '\n';
            exit(0);
        }
    }
    cout << ans << '\n';
}




Measured memory_bytes: 6032