#include <bits/stdc++.h>
using namespace std;
using LL = long long;
using ULL = unsigned long long;
const double PI = acos(-1);
template <class T>
constexpr T INF() {
  return ::std::numeric_limits<T>::max();
}
template <class T>
constexpr T HINF() {
  return INF<T>() / 2;
}
template <typename T_char>
T_char TL(T_char cX) {
  return tolower(cX);
};
template <typename T_char>
T_char TU(T_char cX) {
  return toupper(cX);
};
const int vy[] = {-1, -1, -1, 0, 1, 1, 1, 0},
          vx[] = {-1, 0, 1, 1, 1, 0, -1, -1};
const int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0};
int popcnt(unsigned long long n) {
  int cnt = 0;
  for (int i = 0; i < 64; i++)
    if ((n >> i) & 1) cnt++;
  return cnt;
}
int d_sum(LL n) {
  int ret = 0;
  while (n > 0) {
    ret += n % 10;
    n /= 10;
  }
  return ret;
}
int d_cnt(LL n) {
  int ret = 0;
  while (n > 0) {
    ret++;
    n /= 10;
  }
  return ret;
}
LL gcd(LL a, LL b) {
  if (b == 0) return a;
  return gcd(b, a % b);
};
LL lcm(LL a, LL b) {
  LL g = gcd(a, b);
  return a / g * b;
};
struct INIT {
  INIT() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);
    cout << fixed << setprecision(13);
  }
} INIT;
struct edge {
  LL to, cost;
};
int start;
LL d[100100];
vector<edge> vec[100100];
void dijkstra() {
  for (int i = 0; i < 100100; i++) d[i] = HINF<LL>();
  priority_queue<pair<int, int>, std::vector<pair<int, int>>,
                 std::greater<pair<int, int>>>
      pq;
  d[start] = 0;
  pq.push(make_pair(d[start], start));
  while (!pq.empty()) {
    pair<int, int> p = pq.top();
    pq.pop();
    int i = p.second;
    if (d[i] < p.first) continue;
    for (int k = 0; k < (int)vec[i].size(); k++) {
      edge e = vec[i][k];
      if (d[e.to] <= d[i] + e.cost) continue;
      d[e.to] = d[i] + e.cost;
      pq.push(make_pair(d[e.to], e.to));
    }
  }
}
int n, m;
int l, r, c;
int main() {
  cin >> n >> m;
  for (LL i = (0); i < (m); i++) {
    cin >> l >> r >> c;
    l--, r--;
    vec[l].emplace_back(edge{r, c});
  }
  for (int i = 1; i < n; i++) {
    vec[i].emplace_back(edge{i - 1, 0});
  }
  dijkstra();
  LL ans = d[n - 1];
  cout << (ans == HINF<LL>() ? -1 : ans) << endl;
}
