#include <bits/stdc++.h>
#include <math.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
const ll mod = 1000000007;
const ll INF = 1001001001;
const ll LINF = 1001001001001001001;

void prvec(vector<ll> vec){
    ll n = vec.size();
    rep(i,n) cout << "i" << " " << vec.at(i) << "\n";
}

void pr2d(vector<vector<ll>> vvec){
    ll h = vvec.size();
    ll w = vvec.at(0).size();
    rep(i,h){
        rep(j,w){
            cout << vvec.at(i).at(j) << " ";
        }
        cout << "\n";
    }
}

int main(){
    ll n ; cin >> n;
    ll prev ; cin >> prev;
    bool NG = false;
    if(prev!=0) NG = true;
    ll ans = 0;
    rep(i,n-1){
        ll x ; cin >> x;
        if(x == prev + 1){
            ans++;
        }else if(x>prev+1){
            NG = true;
        }else{
            ans+=x;
        }
        prev = x;
    }
    if(NG){
        cout << -1 << endl;
    }else{
        cout << ans << endl;
    }
    return 0;
}
#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<vector>
#include<functional>
#include<algorithm>
#include<stdlib.h>
#include<string>
#include<string.h>
#define _USE_MATH_DEFINES
#include<math.h>
#include<deque>
#include<set>
#include<map>
#include<queue>
#include<list>
#include<iostream>
#include <bitset>
using namespace std;
typedef long long ll;
#define rep(i,a,b) for(auto i=a;i<b;i++)
#define all(_x) _x.begin(), _x.end()
#define r_sort(_x) sort(_x, std::greater<int>())
#define vec_cnt(_a, _n) (upper_bound(all(_a), _n) - lower_bound(all(_a), _n))
#define vec_unique(_a) _a.erase(std::unique(all(_a)), _a.end());
#define vvec vector<vector<ll>>
ll gcd(ll a, ll b) { return a % b == 0 ? b : gcd(b, a % b); }
ll lcm(ll a, ll b) { return (a / gcd(a, b)) * b; }
#define INF 1 << 30
const int mod = 1000000007;
ll power(ll x, ll p) { ll a = 1; while (p > 0) { if (p % 2 == 0) { x *= x; p /= 2; } else { a *= x; p--; } }return a; }
ll mpower(ll x, ll p) { ll a = 1; while (p > 0) { if (p % 2 == 0) { x = x * x % mod; p /= 2; } else { a = a * x % mod; p--; } }return a; }
ll ac(ll n, ll k) { ll a = 1; rep(i, 1, k) { a *= n - i + 1; a /= i; }return a; }
ll mc(ll n, ll m) { ll k = 1, l = 1; rep(i, n - m + 1, n + 1) k = k * i % mod; rep(i, 1, m + 1) l = l * i % mod; l = mpower(l, mod - 2); return k * l % mod; }
typedef struct { ll n, m; } p2;

int main()
{
	int N;
	cin >> N;
	vector<int> A(N);
	ll ans = 0;
	rep(i, 0, N) {
		cin >> A[i];
		if (A[i] > i || i && A[i] - A[i - 1] > 1) {
			puts("-1");
			return 0;
		}
		if (i && A[i] != A[i - 1] + 1) {
			ans += (ll)A[i - 1];
		}
	}
	ans += (ll)A[N-1];
	printf("%lld\n", ans);
	return 0;
}