#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#ifdef DEBUG
#include "./CompetitiveProgrammingCpp/Timer.hpp"
#include "./CompetitiveProgrammingCpp/debug.hpp"
#include "./CompetitiveProgrammingCpp/sample.hpp"
#else
#define dump(...)
template <class T>
inline auto d_val(T a, T b) {
  return a;
}
#endif

/* (=＾o＾=) */
#define int ll

/* macro */
#define FOR(i, b, e) for (ll i = (ll)(b); i < (ll)(e); ++i)
#define RFOR(i, b, e) for (ll i = (ll)(e - 1); i >= (ll)(b); --i)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) RFOR(i, 0, n)
#define REPC(x, c) for (const auto& x : (c))
#define REPI2(it, b, e) for (auto it = (b); it != (e); ++it)
#define REPI(it, c) REPI2(it, (c).begin(), (c).end())
#define RREPI(it, c) REPI2(it, (c).rbegin(), (c).rend())
#define REPI_ERACE2(it, b, e) for (auto it = (b); it != (e);)
#define REPI_ERACE(it, c) REPI_ERACE2(it, (c).begin(), (c).end())
#define ALL(x) (x).begin(), (x).end()
#define cauto const auto&
/* macro func */
template <class T>
inline auto sort(T& t) {
  std::sort(ALL(t));
}
template <class T>
inline auto rsort(T& t) {
  std::sort((t).rbegin(), (t).rend());
}
template <class T>
inline auto unique(T& t) {
  (t).erase(unique((t).begin(), (t).end()), (t).end());
}
template <class T, class S>
inline auto chmax(T& t, const S& s) {
  if (s > t) {
    t = s;
    return true;
  }
  return false;
}
template <class T, class S>
inline auto chmin(T& t, const S& s) {
  if (s < t) {
    t = s;
    return true;
  }
  return false;
}
inline auto BR() { std::cout << "\n"; }

/* type define */
using ll = long long;
using PAIR = std::pair<ll, ll>;
using VS = std::vector<std::string>;
using VL = std::vector<long long>;
using VVL = std::vector<VL>;
using VVVL = std::vector<VVL>;
using VD = std::vector<double>;
template <class T>
using V = std::vector<T>;

/* using std */
using std::cout;
constexpr char endl = '\n';
using std::bitset;
using std::cin;
using std::list;
using std::map;
using std::multimap;
using std::multiset;
using std::pair;
using std::priority_queue;
using std::queue;
using std::set;
using std::stack;
using std::string;
using std::unordered_map;
using std::unordered_multimap;
using std::unordered_multiset;
using std::unordered_set;
using std::vector;

/* constant value */
constexpr ll MOD = 1000000007;
// constexpr ll MOD = 998244353;

/* Initial processing  */
struct Preprocessing {
  Preprocessing() {
    std::cin.tie(0);
    std::ios::sync_with_stdio(0);
  };
} _Preprocessing;

/* Remove the source of the bug */
signed pow(signed, signed) {
  assert(false);
  return -1;
}

/* define hash */
namespace std {
template <>
class hash<std::pair<ll, ll>> {
 public:
  size_t operator()(const std::pair<ll, ll>& x) const {
    return hash<ll>()(1000000000 * x.first + x.second);
  }
};
}  // namespace std

/* input */
template <class T>
std::istream& operator>>(std::istream& is, vector<T>& vec) {
  for (T& x : vec) is >> x;
  return is;
}

//=============================================================================================

signed main() {
  ll n;
  cin >> n;
  VL v(n);
  cin >> v;

  if (v[0] > 0) {
    cout << -1 << endl;
    return 0;
  }

  ll ans = 0;
  REP(i, n - 1) {
    if (v[i + 1] > v[i] + 1) {
      cout << -1 << endl;
      return 0;
    }
    if (v[i + 1] == v[i] + 1) {
      ++ans;
      continue;
    }
    ans += v[i + 1];
  }
  cout << ans << endl;
}


Measured memory_bytes: 1792