#include <bits/stdc++.h>
using namespace std;
using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int INT_INF = (int)(2e9);
const ll LL_INF = (ll)(2e18);
const int NIL = -1;
static mt19937 _g(time(nullptr));
inline ll randint(ll a, ll b) {
  ll w = (_g() << 31LL) ^ _g();
  return a + w % (b - a + 1);
}
inline void fast_io() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
};
template <typename T>
inline T sign(T x) {
  return T(x > 0) - T(x < 0);
}
template <typename T, typename S>
inline ostream& operator<<(ostream& os, const pair<T, S> p) {
  cout << "[" << p.first << ";" << p.second << "]";
  return os;
}
template <typename T>
inline ostream& operator<<(ostream& os, const vector<T>& v) {
  for (auto el : v) cout << el << " ";
  return os;
}
template <typename T>
inline T fetch() {
  T ret;
  cin >> ret;
  return ret;
}
template <typename T>
inline vector<T> fetch_vec(int sz) {
  vector<T> ret(sz);
  for (auto& elem : ret) cin >> elem;
  return ret;
}
ll solve() {
  int N, M;
  std::cin >> N >> M;
  std::vector<ll> v(N + 1, std::numeric_limits<int>::max());
  v[1] = 0;
  vector<vector<int>> segments(M, vector<int>(3));
  for (int i = 0; i < M; ++i) {
    int L, R, C;
    std::cin >> L >> R >> C;
    segments[i][0] = R;
    segments[i][1] = L;
    segments[i][2] = C;
  }
  std::sort(segments.begin(), segments.end());
  for (int i = 0; i < M; ++i) {
    ll minStart = v[segments[i][1]];
    for (int j = segments[i][1] + 1; j < segments[i][0]; ++j)
      minStart = std::min(minStart, v[j]);
    v[segments[i][0]] = std::min(v[segments[i][0]], minStart + segments[i][2]);
  }
  if (v[N] == std::numeric_limits<int>::max()) return -1;
  return v[N];
}
int main() {
  std::cout << solve() << std::endl;
  return 0;
}
