#include <bits/stdc++.h>
using namespace std;

// boost
#include <boost/multiprecision/cpp_int.hpp>
using boost::multiprecision::cpp_int;

using ll = long long;
using ld = long double;

using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vb = vector<bool>;
using vs = vector<string>;
using vld = vector<ld>;
using vvld = vector<vld>;

typedef pair<ll, ll> P;

#define bit(n) (1LL << (n))

//#define int long long

#define all(v) v.begin(), v.end()

#define rep(i, n) for (ll i = 0; i < n; i++)
#define REP(i, n) for (ll i = 1; i < n; i++)

#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define FORm(i, m) for (auto i = m.begin(); i != m.end(); i++)

template <class T>
inline void chmax(T& a, T b) {
  a = std::max(a, b);
}
template <class T>
inline void chmin(T& a, T b) {
  a = std::min(a, b);
}

#define mod (ll)(1e9 + 7)
// #define mod (998244353ll)

const long long INF = 1LL << 60;

template <typename T>
class SegTree {
  int n;
  vector<T> data;
  T default_value;
  function<T(T a, T b)> operation;
  function<T(T prev_val, T new_val)> _update;

  // [a,b)
  T query(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l) {
      return default_value;
    }

    if (a <= l && r <= b) {
      return data[k];
    } else {
      T vl = query(a, b, 2 * k + 1, l, (l + r) / 2);
      T vr = query(a, b, 2 * k + 2, (l + r) / 2, r);
      return operation(vl, vr);
    }
  }

 public:
  SegTree(int _n, T _default_value, function<T(T a, T b)> _operation,
          function<T(T prev_val, T new_val)> update_function)
    : default_value(_default_value), operation(_operation), _update(update_function) {
    n = 1;
    while (n < _n) {
      n <<= 1;
    }
    data = vector<T>(2 * n - 1, default_value);
  }

  SegTree(vector<T> initial_value, T _default_value, function<T(T a, T b)> _operation,
          function<T(T prev_val, T new_val)> update_function)
    : default_value(_default_value), operation(_operation), _update(update_function) {
    ll sz = initial_value.size();
    n = 1;
    while (n < sz) {
      n <<= 1;
    }
    data = vector<T>(2 * n - 1, default_value);
    for (int i = 0; i < sz; i++) {
      data[n - 1 + i] = initial_value[i];
    }
    for (int i = n - 2; i >= 0; i--) {
      data[i] = operation(data[i * 2 + 1], data[i * 2 + 2]);
    }
  }

  // update i-th value (0-indexed)
  void update(int i, T x) {
    i += n - 1;
    data[i] = _update(data[i], x);
    while (i > 0) {
      i = (i - 1) / 2;
      data[i] = operation(data[i * 2 + 1], data[i * 2 + 2]);
    }
  }

  // execute query of section [a, b)
  T query(int a, int b) {
    return query(a, b, 0, 0, n);
  }

  // get i-th value
  T operator[](int i) {
    return data[i + n - 1];
  }
};

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(20);

  ll n, m;
  cin >> n >> m;

  using tup = tuple<ll, ll, ll>;
  vector<tup> path;

  rep(i, m) {
    ll l, r, c;
    cin >> l >> r >> c;
    l--;
    r--;
    path.emplace_back(l, r, c);
  }

  sort(all(path));

  // get min
  SegTree<ll> st(
    n, INF, [](ll a, ll b) { return min(a, b); }, [](ll prev_val, ll new_val) { return new_val; });

  st.update(0, 0);

  for (const auto& p : path) {
    ll l, r, c;
    l = get<0>(p);
    r = get<1>(p);
    c = get<2>(p);

    ll co = st.query(l, r);
    co += c;
    if (st[r] > co) {
      st.update(r, co);
    }
  }

  ll ans = st[n - 1];
  if (ans >= INF) {
    ans = -1;
  }
  cout << ans << endl;

  return 0;
}


Measured memory_bytes: 7664