/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author Gosu_Hiroo
 */

#include <iostream>
#include <fstream>

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using PII = pair<int,int>;
using VI = vector<int>;
using VVI = vector<vector<int>>;
using VVVI = vector<vector<vector<int>>>;
#define V2D(size_1, size_2, value)vector<vector<int>>(size_1, vector<int>(size_2, value))
#define V3D(size_1, size_2, size_3, value)vector<vector<vector<int>>>(size_1, vector<vector<int>>(size_2, vector<int>(size_3, value)))

#define TR(container, it) \
        for (auto it = container.begin(); it != container.end(); it++)
#define IN(c, x) ((c).find(x) != (c).end())   //O(log n)
#define IN_L(c, x) (find((c).begin(),(c).end(),x) != (c).end()) //O(n)
#define FOR(i, _begin, _end) for (__typeof(_end) end = _end, begin = _begin,  i = (begin) - ((begin) > (end)); i != (end) - ((begin) > (end)); i += 1 - 2 * ((begin) > (end)))
#define REP(i, end) for (__typeof(end) i = 0, _len = (end); i < (_len); i += 1)
#define ALL(x) (x).begin(),(x).end()
#define SZ(x) ((int)(x).size())
#define F first
#define S second
#define y0 y3487465
#define y1 y8687969
#define j0 j1347829
#define j1 j234892
#define MOD(x, m) ((((x) % (m)) + (m)) % (m))
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );
#define READ ({int t;cin >> t;t;})
#define EB emplace_back
#define fcout cout << fixed << setprecision(12)
#define fcerr cerr << fixed << setprecision(12)
#define print(x) cout << (x) << endl
# define BYE(a) do { cout << (a) << endl; return ; } while (false)

#ifdef DEBUG
#define ERR(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); _err(cerr,_it, args); }
#define DBG(x) cerr << #x << " is " << x << endl;
#else
#define DBG(x) {};
#define ERR(args...) {};
#endif
void _err(std::ostream& cerr,istream_iterator<string> it) {cerr << endl;}
template<typename T, typename... Args>
void _err(std::ostream& cerr, istream_iterator<string> it, T a, Args... args) {
    cerr << *it << " = " << a << "  ";
    _err(cerr,++it, args...);
}

const double pi = 2 * acos(.0);
const int inf = 0x3f3f3f3f;
const ll mod = (ll) (1e9) + 7;
//const ll mod = (ll) 998244353 ;
const double eps = 1e-10;

template<typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
    std::fill((T *) array, (T *) (array + N), val);
}


template <typename T>
ostream& operator << (ostream& os, const vector<T> V) {
    os << "[";
    for (int i = 0; i < V.size() - 1; ++i) {
        os << V[i] << " ";
    }
    if(!V.empty())os << V.back();
    os << "]";
    return os;
}
;
template <typename T, typename U>
ostream& operator << (ostream& os, const pair<T,U> P) {
    os << "(";
    os << P.first << "," << P.second;
    os << ")";
    return os;
}

template <typename T>
ostream& operator << (ostream& os, const set<T> V) {
    os << "{";
    if(!V.empty()){
        auto it = V.begin();
        for (int i = 0; i < V.size() -1; ++i) {
            os << *it << " ";
            it++;
        }
        os << *it;
    }
    os << "}";
    return os;
}

template <typename T>
ostream& operator << (ostream& os, const deque<T> V) {
    os << "[";
    for (int i = 0; i < V.size() - 1; ++i) {
        os << V[i] << "->";
    }
    if(!V.empty())os << V.back();
    os << "]";
    return os;
}

template<class T>bool Chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool Chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#define int ll

class CSequenceGrowingEasy {
public:

void solve(std::istream& cin, std::ostream& cout, std::ostream& cerr) {
    int N;
    cin >> N;
    VI a(N);
    REP(i,N)cin >> a[i];
    if(a[0])BYE("-1");
    REP(i,N-1)if(a[i+1] > a[i] + 1)BYE("-1");
    int ans = 0;
    int ind = 0, curr = 0;
    while(ind < N){
        while(a[ind] == curr && ind < N){
            ind ++, curr++;
        }
        ERR(curr)
        ans += curr-1;
        curr = a[ind];
    }
    print(ans);

}


};


#undef int
int main() {

	CSequenceGrowingEasy solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
    std::ostringstream err;
	in.tie(0); ios::sync_with_stdio(0);
//	solver.solve(in, out);
    solver.solve(in, out,err);
	return 0;
}


Measured memory_bytes: 1024