#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
using f80 = long double;
using vi32 = vector<int>;
using vi64 = vector<i64>;
using vf80 = vector<f80>;
using vstr = vector<string>;
inline void yes() {
  cout << "Yes" << endl;
  exit(0);
}
inline void no() {
  cout << "No" << endl;
  exit(0);
}
inline i64 gcd(i64 a, i64 b) {
  if (min(a, b) == 0) return max(a, b);
  if (a % b == 0) return b;
  return gcd(b, a % b);
}
inline i64 lcm(i64 a, i64 b) {
  if (min(a, b) == 0) return max(a, b);
  return a / gcd(a, b) * b;
}
template <typename T>
class pqasc : public priority_queue<T, vector<T>, greater<T>> {};
template <typename T>
class pqdesc : public priority_queue<T, vector<T>, less<T>> {};
template <typename T>
inline void amax(T &x, T y) {
  x = max(x, y);
}
template <typename T>
inline void amin(T &x, T y) {
  x = min(x, y);
}
template <typename T>
inline T exp(T x, i64 n, T e = 1) {
  T r = e;
  while (n > 0) {
    if (n & 1) r *= x;
    x *= x;
    n >>= 1;
  }
  return r;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &v) {
  for (auto &x : v) is >> x;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> &v) {
  for (int i = 0; i < int(v.size()); i++) {
    if (i) os << ' ';
    os << v[i];
  }
  return os;
}
void solve();
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout << fixed << setprecision(16);
  solve();
  return 0;
}
void solve() {
  int N, M;
  cin >> N >> M;
  vector<tuple<int, int, int>> lrc;
  for (int i = 0; i < int(M); i++) {
    int l, r, c;
    cin >> l >> r >> c;
    lrc.emplace_back(l, r, c);
  }
  {
    vi32 imos(N);
    for (int i = 0; i < int(M); i++) {
      int l, r, c;
      tie(l, r, c) = lrc[i];
      imos[l]++;
      imos[r]--;
    }
    for (int i = 1; i <= int(N); i++) imos[i] += imos[i - 1];
    bool ok = true;
    for (int i = 1; i <= int(N - 1); i++) {
      if (imos[i] == 0) ok = false;
    }
    if (!ok) {
      cout << -1 << endl;
      return;
    }
  }
  vi32 dist(M);
  vector<vi32> adds(N);
  vector<vi32> rems(N);
  for (int i = 0; i < int(M); i++) {
    int l, r, c;
    tie(l, r, c) = lrc[i];
    l--, r--;
    adds[l].emplace_back(i);
    rems[r].emplace_back(i);
  }
  i64 ans = 0;
  set<pair<i64, int>> se;
  for (int i = 0; i < int(N); i++) {
    if (i != 0) {
      for (auto &s : se) {
        ans = s.first;
        break;
      }
    }
    for (auto &j : adds[i]) {
      int l, r, c;
      tie(l, r, c) = lrc[j];
      se.emplace(ans + c, j);
      dist[j] = ans + c;
    }
    for (auto &j : rems[i]) {
      se.erase(make_pair(dist[j], j));
    }
  }
  cout << ans << endl;
}
