#include <bits/stdc++.h>
using namespace std;
constexpr int INF = numeric_limits<int>::max() / 2;
constexpr long long LINF = numeric_limits<long long>::max() / 3;
struct Double {
  double d;
  explicit Double(double x) : d(x) {}
};
ostream& operator<<(ostream& os, const Double x) {
  os << fixed << setprecision(20) << x.d;
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& vec) {
  os << "[";
  for (const auto& v : vec) {
    os << v << ",";
  }
  os << "]";
  return os;
}
template <typename T, typename U>
ostream& operator<<(ostream& os, const map<T, U>& mp) {
  os << "{";
  for (auto& p : mp) {
    os << p << ",";
  }
  os << "}";
  return os;
}
template <typename T, typename U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
  os << "(" << p.first << "," << p.second << ")";
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, const set<T>& st) {
  os << "{";
  for (T v : st) os << v << ",";
  os << "}";
  return os;
}
template <typename T, typename U>
inline void chmax(T& x, U y) {
  if (y > x) x = y;
}
template <typename T, typename U>
inline void chmin(T& x, U y) {
  if (y < x) x = y;
}
using ll = long long;
using ull = unsigned long long;
using pii = std::pair<int, int>;
using vi = std::vector<int>;
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n - 1); }
constexpr double eps = 1e-10;
constexpr ll mod = 1e9 + 7;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
int main() {
  int n, m;
  cin >> n >> m;
  set<int> stl, str;
  vector<tuple<int, int, ll>> v;
  str.insert(0);
  str.insert(n - 1);
  for (int i = 0; i < (int)(m); i++) {
    int l, r;
    cin >> l >> r;
    l--;
    r--;
    ll c;
    cin >> c;
    stl.insert(l);
    str.insert(r);
    v.emplace_back(l, r, c);
  }
  vector<vector<pair<int, ll>>> G(n);
  for (auto& t : v) {
    int l, r;
    ll c;
    tie(l, r, c) = t;
    auto si = stl.upper_bound(l);
    auto second = stl.upper_bound(r);
    for (auto iter = si; iter != second; iter++) {
      G[l].emplace_back(*iter, c);
    }
    G[l].emplace_back(r, c);
  }
  vector<ll> dp(n, LINF);
  dp[0] = 0;
  for (int i = 0; i < n - 1; i++) {
    for (auto& p : G[i]) {
      int v = p.first;
      ll c = p.second;
      dp[v] = min(dp[v], dp[i] + c);
    }
  }
  if (dp[n - 1] == LINF) {
    cout << -1 << endl;
    return 0;
  }
  cout << dp[n - 1] << endl;
}
