#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
using Int = unsigned int;
using llong = long long;
using Llong = unsigned long long;
using ldouble = long double;
using intV = vector<int>;
using llongV = vector<long long>;
using llongVV = vector<vector<long long>>;
using Graph = vector<vector<long long>>;
using costGraph = vector<vector<pair<long long, long long>>>;
const llong INF = 1e17;
class SegmentTree {
 public:
  llongV __data;
  llong __size;
  SegmentTree(llong size) { this->init(size); };
  void init(llong size) {
    llong binSize = 0;
    while ((1 << binSize) < size) {
      binSize++;
    }
    this->__size = (1 << binSize);
    this->__data.resize(this->__size * 2);
    for (llong i = 0; i < this->__size * 2; i++) {
      this->__data[i] = INF;
    }
  };
  void update(llong index, llong value) {
    index += this->__size - 1;
    this->__data[index] = value;
    while (index > 0) {
      index = (index - 1) / 2;
      this->__data[index] =
          min(this->__data[index * 2 + 1], this->__data[index * 2 + 2]);
    }
  };
  llong __query(llong qLeft, llong qRight, llong node, llong left,
                llong right) {
    if (right <= qLeft || qRight <= left) return INF;
    if (qLeft <= left && right <= qRight) return this->__data[node];
    llong leftVal =
        this->__query(qLeft, qRight, node * 2 + 1, left, (left + right) / 2);
    llong rightVal =
        this->__query(qLeft, qRight, node * 2 + 2, (left + right) / 2, right);
    return min(leftVal, rightVal);
  }
  llong query(llong left, llong right) {
    return this->__query(left, right, 0, 0, this->__size);
  };
};
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  llong N, M;
  cin >> N >> M;
  SegmentTree tree(N + 1);
  tree.update(1, 0);
  vector<tuple<llong, llong, llong>> edges(M);
  for (llong i = 0LL; i < llong(M); i++)
    cin >> get<0>(edges[i]) >> get<1>(edges[i]) >> get<2>(edges[i]);
  for (llong i = 0LL; i < llong(2); i++) {
    for (auto a : edges) {
      llong L, R, C;
      tie(L, R, C) = a;
      llong cost = tree.query(L, R + 1) + C;
      tree.update(R, min(cost, tree.query(R, R + 1)));
    }
  }
  llong ans = tree.query(N, N + 1);
  if (ans == INF) ans = -1;
  cout << ans << endl;
  return 0;
}
