#include <bits/stdc++.h>
using namespace std;
const int INF = 1001001001;
const long long LINF = 1001001001001001001ll;
const int MOD = 1000000007;
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;
}
template <typename T, typename E>
struct LazySegmentTree {
  using F = function<T(T, T)>;
  using G = function<T(T, E)>;
  using H = function<E(E, E)>;
  int n, height;
  F f;
  G g;
  H h;
  T ti;
  E ei;
  vector<T> dat;
  vector<E> laz;
  LazySegmentTree(F f, G g, H h, T ti, E ei)
      : f(f), g(g), h(h), ti(ti), ei(ei) {}
  void init(int n_) {
    n = 1;
    height = 0;
    while (n < n_) n <<= 1, height++;
    dat.assign(2 * n, ti);
    laz.assign(2 * n, ei);
  }
  void build(const vector<T>& v) {
    int n_ = v.size();
    init(n_);
    for (int i = 0; i < n_; i++) dat[n + i] = v[i];
    for (int i = n - 1; i; i--)
      dat[i] = f(dat[(i << 1) | 0], dat[(i << 1) | 1]);
  }
  inline T reflect(int k) { return laz[k] == ei ? dat[k] : g(dat[k], laz[k]); }
  inline void eval(int k) {
    if (laz[k] == ei) return;
    laz[(k << 1) | 0] = h(laz[(k << 1) | 0], laz[k]);
    laz[(k << 1) | 1] = h(laz[(k << 1) | 1], laz[k]);
    dat[k] = reflect(k);
    laz[k] = ei;
  }
  inline void thrust(int k) {
    for (int i = height; i; i--) eval(k >> i);
  }
  inline void recalc(int k) {
    while (k >>= 1) dat[k] = f(reflect((k << 1) | 0), reflect((k << 1) | 1));
  }
  void update(int a, int b, E x) {
    thrust(a += n);
    thrust(b += n - 1);
    for (int l = a, r = b + 1; l < r; l >>= 1, r >>= 1) {
      if (l & 1) laz[l] = h(laz[l], x), l++;
      if (r & 1) --r, laz[r] = h(laz[r], x);
    }
    recalc(a);
    recalc(b);
  }
  void set_val(int a, T x) {
    thrust(a += n);
    dat[a] = x;
    laz[a] = ei;
    recalc(a);
  }
  T query(int a, int b) {
    thrust(a += n);
    thrust(b += n - 1);
    T vl = ti, vr = ti;
    for (int l = a, r = b + 1; l < r; l >>= 1, r >>= 1) {
      if (l & 1) vl = f(vl, reflect(l++));
      if (r & 1) vr = f(reflect(--r), vr);
    }
    return f(vl, vr);
  }
  template <typename C>
  int find(int st, C& check, T& acc, int k, int l, int r) {
    if (l + 1 == r) {
      acc = f(acc, reflect(k));
      return check(acc) ? k - n : -1;
    }
    eval(k);
    int m = (l + r) >> 1;
    if (m <= st) return find(st, check, acc, (k << 1) | 1, m, r);
    if (st <= l && !check(f(acc, dat[k]))) {
      acc = f(acc, dat[k]);
      return -1;
    }
    int vl = find(st, check, acc, (k << 1) | 0, l, m);
    if (~vl) return vl;
    return find(st, check, acc, (k << 1) | 1, m, r);
  }
  template <typename C>
  int find(int st, C& check) {
    T acc = ti;
    return find(st, check, acc, 1, 0, n);
  }
};
int main() {
  int N, M;
  cin >> N >> M;
  int L, R;
  long long C;
  vector<pair<int, pair<int, long long> > > RLC(M);
  for (int i = 0; i < (M); ++i) {
    cin >> L >> R >> C;
    RLC[i].first = R - 1;
    RLC[i].second.first = L - 1;
    RLC[i].second.second = C;
  }
  sort((RLC).begin(), (RLC).end());
  vector<long long> d(N, LINF);
  int iter = 0;
  d[0] = 0;
  for (int i = 0; i < (M); ++i) {
    long long l = RLC[i].second.first, r = RLC[i].first;
    if (d[l] == LINF) continue;
    long long temp = d[l] + RLC[i].second.second;
    for (int j = iter; j <= (r); ++j) {
      chmin(d[j], temp);
    }
    iter = r;
  }
  if (d[N - 1] == LINF)
    cout << -1 << endl;
  else
    cout << d[N - 1] << endl;
}
