#include <bits/stdc++.h>
using namespace std;
using P = pair<long long, long long>;
using WeightedGraph = vector<vector<P>>;
using UnWeightedGraph = vector<vector<long long>>;
using Real = long double;
using Point = complex<Real>;
using Vector2d = complex<Real>;
const long long INF = 1LL << 60;
const long long MOD = 1000000007;
const double EPS = 1e-15;
const double PI = 3.14159265358979323846;
template <typename T>
long long getIndexOfLowerBound(vector<T> &v, T x) {
  return lower_bound(v.begin(), v.end(), x) - v.begin();
}
template <typename T>
long long getIndexOfUpperBound(vector<T> &v, T x) {
  return upper_bound(v.begin(), v.end(), x) - v.begin();
}
template <class T>
inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
istream &operator>>(istream &is, Point &p) {
  Real a, b;
  is >> a >> b;
  p = Point(a, b);
  return is;
}
template <typename T, typename U>
istream &operator>>(istream &is, pair<T, U> &p_var) {
  is >> p_var.first >> p_var.second;
  return is;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec) is >> x;
  return is;
}
template <typename T>
istream &operator>>(istream &is, vector<vector<T>> &df) {
  for (long long i = 0; i < df.size(); i++)
    for (long long j = 0; j < df[i].size(); j++) is >> df[i][j];
  return is;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
  cerr << '{';
  os << pair_var.first;
  cerr << ',';
  os << " " << pair_var.second;
  cerr << '}';
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> &vec) {
  cerr << '[';
  for (long long i = 0; i < vec.size(); i++)
    os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  cerr << ']';
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, vector<vector<T>> &df) {
  for (auto &vec : df) os << vec;
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
  cerr << "{";
  for (auto itr = map_var.begin(); itr != map_var.end(); itr++) {
    os << *itr;
    itr++;
    if (itr != map_var.end()) cerr << ", ";
    itr--;
  }
  cerr << "}";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, set<T> &set_var) {
  cerr << "{";
  for (auto itr = set_var.begin(); itr != set_var.end(); itr++) {
    os << *itr;
    itr++;
    if (itr != set_var.end()) cerr << ", ";
    itr--;
  }
  cerr << "}";
  return os;
}
void print() { cout << endl; }
template <class Head, class... Tail>
void print(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail) != 0) cout << " ";
  print(forward<Tail>(tail)...);
}
void dump_func() { cerr << '#' << endl; }
template <typename Head, typename... Tail>
void dump_func(Head &&head, Tail &&...tail) {
  cerr << head;
  if (sizeof...(Tail) > 0) cerr << ", ";
  dump_func(std::move(tail)...);
}
vector<long long> dijkstra(WeightedGraph &G, long long s = 0) {
  vector<long long> d(G.size(), INF);
  priority_queue<P, vector<P>, greater<P>> que;
  d[s] = 0;
  que.push(P(0, s));
  while (!que.empty()) {
    P p = que.top();
    que.pop();
    long long v = p.second;
    if (d[v] < p.first) continue;
    long long sz = G[v].size();
    for (long long(i) = 0; (i) < (sz); (i)++)
      if (chmin(d[G[v][i].first], d[v] + G[v][i].second))
        que.push(P(d[G[v][i].first], G[v][i].first));
  }
  return d;
}
signed main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  long long n, m;
  cin >> n >> m;
  WeightedGraph g(n);
  vector<tuple<long long, long long, long long>> edges(m);
  for (long long(i) = 0; (i) < (m); (i)++) {
    long long l, r, c;
    cin >> l >> r >> c;
    l--, r--;
    edges[i] = make_tuple(l, r, c);
  }
  sort(edges.begin(), edges.end());
  priority_queue<long long, vector<long long>, greater<long long>> pq;
  pq.push(0);
  for (long long(i) = 0; (i) < (m); (i)++) {
    long long l = get<0>(edges[i]), r = get<1>(edges[i]), c = get<2>(edges[i]);
    while (!pq.empty()) {
      long long idx = pq.top();
      pq.pop();
      if (idx >= l && idx <= r) {
        g[idx].push_back({r, c});
        pq.push(idx);
        pq.push(r);
        break;
      }
    }
  }
  auto d = dijkstra(g);
  if (d[n - 1] == INF)
    print(-1);
  else
    print(d[n - 1]);
}
