//#define NDEBUG
#include <algorithm>
#include <cstddef>
#include <cstdint>
#include <iostream>
#include <vector>

namespace n91 {

  using i8 = std::int_fast8_t;
  using i32 = std::int_fast32_t;
  using i64 = std::int_fast64_t;
  using u8 = std::uint_fast8_t;
  using u32 = std::uint_fast32_t;
  using u64 = std::uint_fast64_t;
  using isize = std::ptrdiff_t;
  using usize = std::size_t;

  struct rep {
    struct itr {
      usize i;
      constexpr itr(const usize i) noexcept : i(i) {}
      void operator++() noexcept { ++i; }
      constexpr usize operator*() const noexcept { return i; }
      constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
    };
    const itr f, l;
    constexpr rep(const usize f, const usize l) noexcept
      : f(std::min(f, l)), l(l) {}
    constexpr auto begin() const noexcept { return f; }
    constexpr auto end() const noexcept { return l; }
  };
  struct revrep {
    struct itr {
      usize i;
      constexpr itr(const usize i) noexcept : i(i) {}
      void operator++() noexcept { --i; }
      constexpr usize operator*() const noexcept { return i; }
      constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
    };
    const itr f, l;
    constexpr revrep(const usize f, const usize l) noexcept
      : f(l - 1), l(std::min(f, l) - 1) {}
    constexpr auto begin() const noexcept { return f; }
    constexpr auto end() const noexcept { return l; }
  };
  template <class T> auto md_vec(const usize n, const T& value) {
    return std::vector<T>(n, value);
  }
  template <class... Args> auto md_vec(const usize n, Args... args) {
    return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
  }
  template <class T> constexpr T difference(const T& a, const T& b) noexcept {
    return a < b ? b - a : a - b;
  }
  template <class T> void chmin(T& a, const T& b) noexcept {
    if (b < a)
      a = b;
  }
  template <class T> void chmax(T& a, const T& b) noexcept {
    if (a < b)
      a = b;
  }
  template <class T> T scan() {
    T ret;
    std::cin >> ret;
    return ret;
  }

} // namespace n91

#include <algorithm>
#include <iostream>
#include <limits>
#include <set>
#include <utility>

namespace n91 {

  void main_() {
    /*
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    //*/
    constexpr u64 Inf = std::numeric_limits<u64>::max() / 10;
    const usize n = scan<usize>();
    const usize m = scan<usize>();
    struct iv_t {
      usize l, r;
      u64 c;
    };
    std::vector<std::vector<iv_t>> ivl(n), ivr(n);
    for (const usize i : rep(0, m)) {
      const usize l = scan<usize>() - 1;
      const usize r = scan<usize>() - 1;
      const u64 c = scan<u64>();
      ivl[l].push_back({ l, r, c });
      ivr[r].push_back({ l, r, c });
    }
    std::multiset<u64> ms;
    std::vector<u64> dist(n, Inf);
    dist[0] = 0;
    for (const usize i : rep(0, n)) {
      if (!ms.empty()) {
        dist[i] = *ms.cbegin();
      }
      for (const auto& e : ivr[i]) {
        ms.erase(ms.find(dist[e.l] + e.c));
      }
      for (const auto& e : ivl[i]) {
        ms.insert(dist[i] + e.c);
      }
    }
    const u64 ans = dist[n - 1];
    if (ans < Inf) {
      std::cout << ans << std::endl;
    }
    else {
      std::cout << "-1" << std::endl;
    }
  }

} // namespace n91

int main() {
  n91::main_();
  return 0;
}


Measured memory_bytes: 13952