#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;
using vl = vector<ll>;
using vll = vector<vl>;
using vpll = vector<pll>;
int ctoi(char c) {
  switch (c) {
    case '0':
      return 0;
    case '1':
      return 1;
    case '2':
      return 2;
    case '3':
      return 3;
    case '4':
      return 4;
    case '5':
      return 5;
    case '6':
      return 6;
    case '7':
      return 7;
    case '8':
      return 8;
    case '9':
      return 9;
    default:
      return 0;
  }
}
bool pairCompare(const pll firstElof, pll secondElof) {
  return firstElof.first < secondElof.first;
}
ll nod(ll F) {
  ll keta = 1;
  while (F / 10 > 0) {
    keta++;
    F /= 10;
  }
  return keta;
}
ll gcd(ll x, ll y) {
  ll r;
  if (x < y) {
    swap(x, y);
  }
  while (y > 0) {
    r = x % y;
    x = y;
    y = r;
  }
  return x;
}
ll lcm(ll x, ll y) { return x * y / gcd(x, y); }
ll isPrime(ll x) {
  ll i;
  if (x < 2) {
    return 0;
  } else if (x == 2) {
    return 1;
  } else if (x % 2 == 0) {
    return 0;
  } else {
    for (i = 3; i * i <= x; i += 2) {
      if (x % i == 0) {
        return 0;
      }
    }
    return 1;
  }
}
void eratos(vl isPrime) {
  ll i, j;
  for (i = 0; i < isPrime.size(); i++) {
    isPrime[i] = 1;
  }
  isPrime[0] = 0;
  isPrime[1] = 0;
  for (i = 2; i * i <= isPrime.size() - 1; i++) {
    if (isPrime[i] == 1) {
      j = i * 2;
      while (j <= isPrime.size() - 1) {
        isPrime[j] = 0;
        j = j + i;
      }
    }
  }
}
ll modinv(ll a, ll m) {
  ll b = m, u = 1, v = 0;
  while (b) {
    ll t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0) u += m;
  return u;
}
ll alphabet(char C) {
  ll b = 0;
  ll key = 0;
  if (C == 'a') {
    return b;
  }
  b++;
  if (C == 'b') {
    return b;
  }
  b++;
  if (C == 'c') {
    return b;
  }
  b++;
  if (C == 'd') {
    return b;
  }
  b++;
  if (C == 'e') {
    return b;
  }
  b++;
  if (C == 'f') {
    return b;
  }
  b++;
  if (C == 'g') {
    return b;
  }
  b++;
  if (C == 'h') {
    return b;
  }
  b++;
  if (C == 'i') {
    return b;
  }
  b++;
  if (C == 'j') {
    return b;
  }
  b++;
  if (C == 'k') {
    return b;
  }
  b++;
  if (C == 'l') {
    return b;
  }
  b++;
  if (C == 'm') {
    return b;
  }
  b++;
  if (C == 'n') {
    return b;
  }
  b++;
  if (C == 'o') {
    return b;
  }
  b++;
  if (C == 'p') {
    return b;
  }
  b++;
  if (C == 'q') {
    return b;
  }
  b++;
  if (C == 'r') {
    return b;
  }
  b++;
  if (C == 's') {
    return b;
  }
  b++;
  if (C == 't') {
    return b;
  }
  b++;
  if (C == 'u') {
    return b;
  }
  b++;
  if (C == 'v') {
    return b;
  }
  b++;
  if (C == 'w') {
    return b;
  }
  b++;
  if (C == 'x') {
    return b;
  }
  b++;
  if (C == 'y') {
    return b;
  }
  b++;
  if (C == 'z') {
    return b;
  }
  return -1;
}
void bitSearch(ll n) {
  ll i, j;
  for (i = 0; i < pow(2, n); i++) {
    ll p = i;
    for (j = 0; j < n; j++) {
      cout << p % 2;
      p /= 2;
    }
    cout << endl;
  }
}
void dfs(ll now, ll C) {
  stack<ll> S;
  S.push(now);
  ll u = S.top();
  S.pop();
}
void bfs(ll now) {
  queue<ll> Q;
  Q.push(now);
  ll u;
  while (!Q.empty()) {
    u = Q.front();
    Q.pop();
  }
}
ll i, j, k, l, m, n, N, M, K, H, W, L;
string S, T;
ll MOD = 1000000007;
ll ans = 0;
ll INF = (1 << 21);
int main() {
  cin >> N >> M;
  vector<vpll> path(N);
  for (i = 0; i < M; i++) {
    ll l, r, c;
    cin >> l >> r >> c;
    path[l - 1].push_back(make_pair(r - 1, c));
  }
  for (i = 1; i < N; i++) {
    path[i].push_back(make_pair(i - 1, 0));
  }
  ll minv;
  vl D(N), color(N);
  for (i = 0; i < N; i++) {
    D[i] = INF;
    color[i] = 0;
  }
  D[0] = 0;
  color[0] = 1;
  while (true) {
    minv = INF;
    ll u = -1;
    for (i = 0; i < N; i++) {
      if (minv > D[i] && color[i] != 2) {
        u = i;
        minv = D[i];
      }
    }
    if (u == -1) {
      break;
    }
    color[u] = 2;
    for (ll v = 0; v < path[u].size(); v++) {
      if (color[path[u][v].first] != 2) {
        if (D[path[u][v].first] > D[u] + path[u][v].second) {
          D[path[u][v].first] = D[u] + path[u][v].second;
          color[v] = 1;
        }
      }
    }
  }
  cout << (D[N - 1] == INF ? -1 : D[N - 1]);
}
