#include <bits/stdc++.h>
using namespace std;
using ll = long long;
constexpr ll inf = (long long)1e18;
static const ll N = 1e6 + 10;
ll max_v[4 * N], smax_v[4 * N], max_c[4 * N];
ll min_v[4 * N], smin_v[4 * N], min_c[4 * N];
ll sum[4 * N];
class SegmentTree {
  ll n, n0;
  void update_node_max(ll k, ll x) {
    sum[k] += (x - max_v[k]) * max_c[k];
    if (max_v[k] == min_v[k]) {
      max_v[k] = min_v[k] = x;
    } else if (max_v[k] == smin_v[k]) {
      max_v[k] = smin_v[k] = x;
    } else {
      max_v[k] = x;
    }
  }
  void update_node_min(ll k, ll x) {
    sum[k] += (x - min_v[k]) * min_c[k];
    if (max_v[k] == min_v[k]) {
      max_v[k] = min_v[k] = x;
    } else if (smax_v[k] == min_v[k]) {
      min_v[k] = smax_v[k] = x;
    } else {
      min_v[k] = x;
    }
  }
  void push(ll k) {
    if (max_v[k] < max_v[2 * k + 1]) {
      update_node_max(2 * k + 1, max_v[k]);
    }
    if (min_v[2 * k + 1] < min_v[k]) {
      update_node_min(2 * k + 1, min_v[k]);
    }
    if (max_v[k] < max_v[2 * k + 2]) {
      update_node_max(2 * k + 2, max_v[k]);
    }
    if (min_v[2 * k + 2] < min_v[k]) {
      update_node_min(2 * k + 2, min_v[k]);
    }
  }
  void update(ll k) {
    sum[k] = sum[2 * k + 1] + sum[2 * k + 2];
    if (max_v[2 * k + 1] < max_v[2 * k + 2]) {
      max_v[k] = max_v[2 * k + 2];
      max_c[k] = max_c[2 * k + 2];
      smax_v[k] = max(max_v[2 * k + 1], smax_v[2 * k + 2]);
    } else if (max_v[2 * k + 1] > max_v[2 * k + 2]) {
      max_v[k] = max_v[2 * k + 1];
      max_c[k] = max_c[2 * k + 1];
      smax_v[k] = max(smax_v[2 * k + 1], max_v[2 * k + 2]);
    } else {
      max_v[k] = max_v[2 * k + 1];
      max_c[k] = max_c[2 * k + 1] + max_c[2 * k + 2];
      smax_v[k] = max(smax_v[2 * k + 1], smax_v[2 * k + 2]);
    }
    if (min_v[2 * k + 1] < min_v[2 * k + 2]) {
      min_v[k] = min_v[2 * k + 1];
      min_c[k] = min_c[2 * k + 1];
      smin_v[k] = min(smin_v[2 * k + 1], min_v[2 * k + 2]);
    } else if (min_v[2 * k + 1] > min_v[2 * k + 2]) {
      min_v[k] = min_v[2 * k + 2];
      min_c[k] = min_c[2 * k + 2];
      smin_v[k] = min(min_v[2 * k + 1], smin_v[2 * k + 2]);
    } else {
      min_v[k] = min_v[2 * k + 1];
      min_c[k] = min_c[2 * k + 1] + min_c[2 * k + 2];
      smin_v[k] = min(smin_v[2 * k + 1], smin_v[2 * k + 2]);
    }
  }
  void _update_min(ll x, ll a, ll b, ll k, ll l, ll r) {
    if (b <= l || r <= a || max_v[k] <= x) {
      return;
    }
    if (a <= l && r <= b && smax_v[k] < x) {
      update_node_max(k, x);
      return;
    }
    push(k);
    _update_min(x, a, b, 2 * k + 1, l, (l + r) / 2);
    _update_min(x, a, b, 2 * k + 2, (l + r) / 2, r);
    update(k);
  }
  void _update_max(ll x, ll a, ll b, ll k, ll l, ll r) {
    if (b <= l || r <= a || x <= min_v[k]) {
      return;
    }
    if (a <= l && r <= b && x < smin_v[k]) {
      update_node_min(k, x);
      return;
    }
    push(k);
    _update_max(x, a, b, 2 * k + 1, l, (l + r) / 2);
    _update_max(x, a, b, 2 * k + 2, (l + r) / 2, r);
    update(k);
  }
  ll _query_max(ll a, ll b, ll k, ll l, ll r) {
    if (b <= l || r <= a) {
      return -inf;
    }
    if (a <= l && r <= b) {
      return max_v[k];
    }
    push(k);
    ll lv = _query_max(a, b, 2 * k + 1, l, (l + r) / 2);
    ll rv = _query_max(a, b, 2 * k + 2, (l + r) / 2, r);
    return max(lv, rv);
  }
  ll _query_min(ll a, ll b, ll k, ll l, ll r) {
    if (b <= l || r <= a) {
      return inf;
    }
    if (a <= l && r <= b) {
      return min_v[k];
    }
    push(k);
    ll lv = _query_min(a, b, 2 * k + 1, l, (l + r) / 2);
    ll rv = _query_min(a, b, 2 * k + 2, (l + r) / 2, r);
    return min(lv, rv);
  }
  ll _query_sum(ll a, ll b, ll k, ll l, ll r) {
    if (b <= l || r <= a) {
      return 0;
    }
    if (a <= l && r <= b) {
      return sum[k];
    }
    push(k);
    ll lv = _query_sum(a, b, 2 * k + 1, l, (l + r) / 2);
    ll rv = _query_sum(a, b, 2 * k + 2, (l + r) / 2, r);
    return lv + rv;
  }

 public:
  SegmentTree(ll n, vector<ll>& a) : n(n) {
    n0 = 1;
    while (n0 < n) n0 <<= 1;
    for (ll i = 0; i < n; ++i) {
      max_v[n0 - 1 + i] = min_v[n0 - 1 + i] = sum[n0 - 1 + i] = (a[i]);
      smax_v[n0 - 1 + i] = -inf;
      smin_v[n0 - 1 + i] = inf;
      max_c[n0 - 1 + i] = min_c[n0 - 1 + i] = 1;
    }
    for (ll i = n; i < n0; ++i) {
      max_v[n0 - 1 + i] = smax_v[n0 - 1 + i] = -inf;
      min_v[n0 - 1 + i] = smin_v[n0 - 1 + i] = inf;
      max_c[n0 - 1 + i] = min_c[n0 - 1 + i] = 0;
    }
    for (ll i = n0 - 2; i >= 0; i--) update(i);
  }
  void update_min(ll a, ll b, ll x) { return _update_min(x, a, b, 0, 0, n0); }
  void update_max(ll a, ll b, ll x) { return _update_max(x, a, b, 0, 0, n0); }
  ll query_max(ll a, ll b) { return _query_max(a, b, 0, 0, n0); }
  ll query_min(ll a, ll b) { return _query_min(a, b, 0, 0, n0); }
  ll query_sum(ll a, ll b) { return _query_sum(a, b, 0, 0, n0); }
};
signed main() {
  ll N;
  cin >> N;
  ll M;
  cin >> M;
  using tp = tuple<ll, ll, ll>;
  vector<tp> data;
  for (ll i = 0; i < M; i++) {
    ll l, r, c;
    cin >> l >> r >> c;
    data.emplace_back(l, r, c);
  }
  sort(data.begin(), data.end());
  vector<ll> v((1e6 + 10), inf);
  auto sgt = SegmentTree((ll)1e6 + 10, v);
  for (auto p : data) {
    ll l, r, c;
    tie(l, r, c) = p;
    if (sgt.query_min(l, r + 1) == inf) {
      if (l == 1) sgt.update_min(l, r + 1, c);
    } else {
      ll mini = sgt.query_min(l, r + 1);
      sgt.update_min(l, r + 1, mini + c);
    }
  }
  cout << (sgt.query_min(N, N + 1) == inf ? -1 : sgt.query_min(N, N + 1))
       << endl;
}
