#include <bits/stdc++.h>
using namespace std;
class SegmentTree {
  const long long inf = 1e18;
  int n, n0;
  long long max_v[4 * 100001], smax_v[4 * 100001], max_c[4 * 100001];
  long long min_v[4 * 100001], smin_v[4 * 100001], min_c[4 * 100001];
  long long sum[4 * 100001];
  long long len[4 * 100001], ladd[4 * 100001], lval[4 * 100001];
  void update_node_max(int k, long long 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;
    }
    if (lval[k] != inf && x < lval[k]) {
      lval[k] = x;
    }
  }
  void update_node_min(int k, long long 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;
    }
    if (lval[k] != inf && lval[k] < x) {
      lval[k] = x;
    }
  }
  void push(int k) {
    if (n0 - 1 <= k) return;
    if (lval[k] != inf) {
      updateall(2 * k + 1, lval[k]);
      updateall(2 * k + 2, lval[k]);
      lval[k] = inf;
      return;
    }
    if (ladd[k] != 0) {
      addall(2 * k + 1, ladd[k]);
      addall(2 * k + 2, ladd[k]);
      ladd[k] = 0;
    }
    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(int 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(long long x, int a, int b, int k, int l, int 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(long long x, int a, int b, int k, int l, int 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);
  }
  void addall(int k, long long x) {
    max_v[k] += x;
    if (smax_v[k] != -inf) smax_v[k] += x;
    min_v[k] += x;
    if (smin_v[k] != inf) smin_v[k] += x;
    sum[k] += len[k] * x;
    if (lval[k] != inf) {
      lval[k] += x;
    } else {
      ladd[k] += x;
    }
  }
  void updateall(int k, long long x) {
    max_v[k] = x;
    smax_v[k] = -inf;
    min_v[k] = x;
    smin_v[k] = inf;
    max_c[k] = min_c[k] = len[k];
    sum[k] = x * len[k];
    lval[k] = x;
    ladd[k] = 0;
  }
  void _add_val(long long x, int a, int b, int k, int l, int r) {
    if (b <= l || r <= a) {
      return;
    }
    if (a <= l && r <= b) {
      addall(k, x);
      return;
    }
    push(k);
    _add_val(x, a, b, 2 * k + 1, l, (l + r) / 2);
    _add_val(x, a, b, 2 * k + 2, (l + r) / 2, r);
    update(k);
  }
  void _update_val(long long x, int a, int b, int k, int l, int r) {
    if (b <= l || r <= a) {
      return;
    }
    if (a <= l && r <= b) {
      updateall(k, x);
      return;
    }
    push(k);
    _update_val(x, a, b, 2 * k + 1, l, (l + r) / 2);
    _update_val(x, a, b, 2 * k + 2, (l + r) / 2, r);
    update(k);
  }
  long long _query_max(int a, int b, int k, int l, int r) {
    if (b <= l || r <= a) {
      return -inf;
    }
    if (a <= l && r <= b) {
      return max_v[k];
    }
    push(k);
    long long lv = _query_max(a, b, 2 * k + 1, l, (l + r) / 2);
    long long rv = _query_max(a, b, 2 * k + 2, (l + r) / 2, r);
    return max(lv, rv);
  }
  long long _query_min(int a, int b, int k, int l, int r) {
    if (b <= l || r <= a) {
      return inf;
    }
    if (a <= l && r <= b) {
      return min_v[k];
    }
    push(k);
    long long lv = _query_min(a, b, 2 * k + 1, l, (l + r) / 2);
    long long rv = _query_min(a, b, 2 * k + 2, (l + r) / 2, r);
    return min(lv, rv);
  }
  long long _query_sum(int a, int b, int k, int l, int r) {
    if (b <= l || r <= a) {
      return 0;
    }
    if (a <= l && r <= b) {
      return sum[k];
    }
    push(k);
    long long lv = _query_sum(a, b, 2 * k + 1, l, (l + r) / 2);
    long long rv = _query_sum(a, b, 2 * k + 2, (l + r) / 2, r);
    return lv + rv;
  }

 public:
  SegmentTree(int n) { SegmentTree(n, nullptr); }
  SegmentTree(int n, long long *a) : n(n) {
    n0 = 1;
    while (n0 < n) n0 <<= 1;
    for (int i = 0; i < 2 * n0; ++i) ladd[i] = 0, lval[i] = inf;
    len[0] = n0;
    for (int i = 0; i < n0 - 1; ++i)
      len[2 * i + 1] = len[2 * i + 2] = (len[i] >> 1);
    for (int i = 0; i < n; ++i) {
      max_v[n0 - 1 + i] = min_v[n0 - 1 + i] = sum[n0 - 1 + i] =
          (a != nullptr ? a[i] : 0);
      smax_v[n0 - 1 + i] = -inf;
      smin_v[n0 - 1 + i] = inf;
      max_c[n0 - 1 + i] = min_c[n0 - 1 + i] = 1;
    }
    for (int 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 (int i = n0 - 2; i >= 0; i--) {
      update(i);
    }
  }
  void update_min(int a, int b, long long x) { _update_min(x, a, b, 0, 0, n0); }
  void update_max(int a, int b, long long x) { _update_max(x, a, b, 0, 0, n0); }
  void add_val(int a, int b, long long x) { _add_val(x, a, b, 0, 0, n0); }
  void update_val(int a, int b, long long x) { _update_val(x, a, b, 0, 0, n0); }
  long long query_max(int a, int b) { return _query_max(a, b, 0, 0, n0); }
  long long query_min(int a, int b) { return _query_min(a, b, 0, 0, n0); }
  long long query_sum(int a, int b) { return _query_sum(a, b, 0, 0, n0); }
};
int main() {
  int n, m;
  cin >> n >> m;
  long long a[n];
  fill(a, a + n, 1LL << 50);
  a[0] = 0;
  pair<int, int> p[m];
  long long c[m];
  for (int i = 0; i < m; i++) {
    cin >> p[i].first >> p[i].second >> c[i];
    p[i].first--;
  }
  sort(p, p + m);
  SegmentTree sg = SegmentTree(n, a);
  for (int i = 0; i < m; i++) {
    long long mn = sg.query_min(p[i].first, p[i].second);
    sg.update_min(p[i].first, p[i].second, mn + c[i]);
  }
  long long ans = sg.query_min(n - 1, n);
  if (ans < (1LL << 50)) {
    cout << ans << endl;
  } else {
    cout << -1 << endl;
  }
  return 0;
}
