// IOI 2021
#include <bits/stdc++.h>
using namespace std;
 
#define endl '\n'
#define ends ' '
#define die(x) return cout << x << endl, 0
#define all(v) v.begin(), v.end()
#define sz(x) (int)(x.size())
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) { cerr << ends << H; debug_out(T...); }
#define debug(...) cerr << "{" << #__VA_ARGS__ << "}:", debug_out(__VA_ARGS__)
typedef long long ll;
typedef pair<int, int> pii;
const int INF = 1e9;
const ll MOD = 1e9 + 7;
 
////////////////////////////////////////////////////////////////////
 
const int N = 2e5 + 5;

int A[N];

int main() {
 
	ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
	mt19937 Rnd(time(0));

	int n; cin >> n;
	for (int i = 0; i < n; i++) cin >> A[i];
	if (A[0] != 0) die(-1);
	for (int i = 1; i < n; i++) if (A[i] - A[i - 1] > 1) die(-1);
	ll res = 0;
	for (int i = 1; i < n; i++) if (A[i] == A[i - 1] + 1) res++; else res += A[i];
	cout << res << endl;

	return 0;
}

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <cmath>
#include <vector>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <queue>
#include <ctime>
#include <cassert>
#include <complex>
#include <string>
#include <cstring>
#include <chrono>
#include <random>
#include <bitset>
#include<iomanip>
#include<functional>
using namespace std;
typedef long long ll;




// a と b の最大公約数を返す関数
long long GCD(long long a, long long b) {
    if (b == 0) return a;
    else return GCD(b, a % b);
}

int main() {
    int N;
    cin >> N;
    map<int, int> A;
    for (int i = 0; i < N; i++) {
        int a; cin >> a;
        A[a]++;
    }
    int ans = 0;
    for (auto x : A) {
        if (x.second % 2 != 0) ans++;
    }
    cout << ans << endl;
}
