#include <bits/stdc++.h>

#if MYDEBUG
#include "lib/cp_debug.hpp"
#else
#define DBG(...) ;
#endif
#if __cplusplus <= 201402L
template <typename T>
T gcd(T a, T b) { return ((a % b == 0) ? b : gcd(b, a % b)); }
template <typename T>
T lcm(T a, T b) { return a / gcd(a, b) * b; }
#endif
using LL = long long;
constexpr LL LINF = 334ll << 53;
constexpr int INF = 15 << 26;
constexpr LL MOD = 1E9 + 7;

namespace Problem {
using namespace std;
struct RangeMin {
  using T = long long;
  T operator()(const T &a, const T &b) { return min(a, b); }
  static constexpr T id() { return T(LINF); };
};

struct RangeSetMin {
  using T = long long;
  T operator()(const T &a, const T &b) { return min(a, b); };
  static constexpr T id() { return T(LINF); };
};

struct Min_SetMin {
  using V = RangeMin;
  using A = RangeSetMin;
  V::T operator()(const V::T &a, const A::T &b, const int &h) { return min(a, b); };
};

template <typename M>
struct LazySegmentTree {
  using Val = typename M::V::T;
  using Del = typename M::A::T;
  typename M::V calc;
  typename M::A composite;
  M act;
  vector<Val> val;
  vector<Del> delay;

  int n, height;
  explicit LazySegmentTree(int size) : n(1), height(0) {
    while (n < size) {
      n <<= 1;
      ++height;
    }
    val = vector<Val>(n << 1, calc.id());
    delay = vector<Del>(n << 1, composite.id());
  }
  explicit LazySegmentTree(vector<Val> &v) : n(1), height(0) {
    while (n < v.size()) {
      n <<= 1;
      ++height;
    }
    val = vector<Val>(n << 1, calc.id());
    delay = vector<Del>(n << 1, composite.id());
    copy(v.begin(), v.end(), val.begin() + n);
    for (int i = n - 1; i > 0; --i) {
      val[i] = calc(val[i * 2], val[i * 2 + 1]);
    }
  }
  void propagate(int k, int h) {
    val[k] = act(val[k], delay[k], h);
    if (h > 0) {
      delay[2 * k] = composite(delay[2 * k], delay[k]);
      delay[2 * k + 1] = composite(delay[2 * k + 1], delay[k]);
    }
    delay[k] = composite.id();
  }
  inline void update(int l, int r, Del v) { update(l, r, v, 1, 0, n, height); }
  void update(int l, int r, Del v, int k, int a, int b, int h) {
    if (l <= a && b <= r) {
      delay[k] = composite(delay[k], v);
      propagate(k, h);
      return;
    }
    propagate(k, h);
    if (b <= l || r <= a) {
      return;
    }  //
    update(l, r, v, 2 * k, a, (a + b) / 2, h - 1);
    update(l, r, v, 2 * k + 1, (a + b) / 2, b, h - 1);
    val[k] = calc(val[2 * k], val[2 * k + 1]);
  }
  inline Val query(int l, int r) { return query(l, r, 1, 0, n, height); }
  Val query(int l, int r, int k, int a, int b, int h) {
    if (b <= l || r <= a) {
      return calc.id();
    }  //
    propagate(k, h);
    if (l <= a && b <= r) {
      return val[k];
    }
    Val vall = query(l, r, 2 * k, a, (a + b) / 2, h - 1);
    Val valr = query(l, r, 2 * k + 1, (a + b) / 2, b, h - 1);
    return calc(vall, valr);
  }
};

class Solver2 {
 public:
  int n, m;
  LazySegmentTree<Min_SetMin> seg;
  vector<int> l, r, p;
  vector<LL> c;
  Solver2(LL n, LL m) : n(n), m(m), seg(n), l(m), r(m), p(m), c(m){};

  void solve() {
    for (int i = 0; i < m; ++i) {
      cin >> l[i] >> r[i] >> c[i];
      l[i]--;
      p[i] = i;
    }
    sort(p.begin(), p.end(), [&](int x, int y) -> bool { return l[x] < l[y] or (l[x] == l[y] and r[x] < r[y]); });

    seg.update(0, 1, 0);

    for (int i = 0; i < m; ++i) {
      int id = p[i];
      LL mini = seg.query(l[id], r[id]);
      seg.update(l[id], r[id], mini + c[id]);
    }
    LL ans = seg.query(n - 1, n);
    if (ans == LINF) {
      cout << -1 << endl;
    } else {
      cout << ans << endl;
    }
  }
};
}  // namespace Problem

int main() {
  std::cin.tie(0);
  std::ios_base::sync_with_stdio(false);
  // std::cout << std::fixed << std::setprecision(12);
  long long n = 0, m;
  std::cin >> n >> m;

  Problem::Solver2 sol(n, m);
  sol.solve();
  return 0;
}


Measured memory_bytes: 6272