/*** Template Begin ***/

#pragma region template

#define USING_NAMESPACE

#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

typedef int64_t i64;
typedef uint64_t u64;

auto init_ = [] {
    std::ios_base::sync_with_stdio(false);
    std::cout << std::fixed;
    return 0;
}();

template <typename T>
inline void in(T &x) {
    std::cin >> x;
}

template <typename T, typename... Ts>
inline void in(T &t, Ts &... ts) {
    std::cin >> t;
    in(ts...);
}

template <typename T, typename U = std::vector<T>>
inline U vin(int n) {
    U v(n);
    for (int i = 0; i < n; ++i) {
        std::cin >> v[i];
    }
    return v;
}

template <typename T, typename U = std::vector<T>, typename V = std::vector<U>>
inline V vin(int h, int w) {
    V vv(h, U(w));
    for (int i = 0; i < h; ++i) {
        for (int j = 0; j < w; ++j) {
            std::cin >> vv[i][j];
        }
    }
    return vv;
}

template <typename T>
inline void out(const T &x) {
    std::cout << x << std::endl;
}

template <char delimiter = ' ', typename T, typename... Ts>
inline void out(const T &t, const Ts &... ts) {
    std::cout << t << delimiter;
    out(ts...);
}

template <char delimiter = ' ', typename T>
inline void vout(const T &v, int n) {
    for (int i = 0; i < n; ++i) {
        if (i) std::cout << delimiter;
        std::cout << v[i];
    }
    std::cout << std::endl;
}

template <char delimiter = ' ', typename T>
inline void vout(const T &v, int h, int w) {
    for (int i = 0; i < h; ++i) {
        for (int j = 0; j < w; ++j) {
            if (j) std::cout << delimiter;
            std::cout << v[i][j];
        }
        std::cout << std::endl;
    }
}

template <typename T, size_t D>
struct multi_vector_type {
    using type = std::vector<typename multi_vector_type<T, D - 1>::type>;
};

template <typename T>
struct multi_vector_type<T, 1> {
    using type = std::vector<T>;
};

template <typename T>
struct multi_vector_type<T, 0> {
    using type = T;
};

template <typename T, size_t D>
using multi_vector = typename multi_vector_type<T, D>::type;

template <typename T, size_t D, class = typename std::enable_if<D == 0>::type>
T make_vector(const T &val = T()) {
    return val;
}

template <typename T, size_t D = 1, typename... Ts,
          class = typename std::enable_if<D != 0>::type>
multi_vector<T, D> make_vector(size_t n, Ts &&... args) {
    return multi_vector<T, D>(n, make_vector<T, D - 1>(args...));
}

namespace detail {

template <typename F>
struct Debug {
    const char *delim_ = "\n";
    F fun;

    Debug(F f) : fun(f) {}

    ~Debug() { fun(delim_); }

    Debug &delim(const char *d) {
        delim_ = d;
        return *this;
    }
};

std::deque<std::string> split(const std::string &s, char c) {
    std::deque<std::string> v;
    std::stringstream ss(s);
    std::string x;
    while (std::getline(ss, x, c)) v.emplace_back(x);
    return v;
}

template <typename T>
void deb(const char *delim, std::deque<std::string> v, T a) {
    std::cerr << v[0].substr(v[0][0] == ' ', v[0].length()) << " = " << a
              << '\n';
    std::cerr << std::flush;
}

template <typename T, typename... Args>
void deb(const char *delim, std::deque<std::string> v, T a, Args... args) {
    std::cerr << v[0].substr(v[0][0] == ' ', v[0].length()) << " = " << a
              << delim;
    v.pop_front();
    deb(delim, std::move(v), args...);
}

template <typename... Args>
auto wrap(std::deque<std::string> v, Args... args) {
    auto f = [=](const char *delim = "\n") { deb(delim, v, args...); };

    return Debug<decltype(f)>(f);
}
}

#define debug(args...) ::detail::wrap(::detail::split(#args, ','), args)

#ifdef USING_BOOST

#include <boost/math/common_factor.hpp>
#include <boost/range.hpp>
#include <boost/range/adaptors.hpp>
#include <boost/range/algorithm.hpp>
#include <boost/range/algorithm_ext.hpp>
#include <boost/range/irange.hpp>
#include <boost/range/numeric.hpp>

inline auto rep(int begin, int end) {
    if (begin > end) {
        return boost::irange(0, 0);
    } else {
        return boost::irange(begin, end);
    }
}

inline auto rep(int begin, int end, int step) {
    if ((step > 0 && begin > end) || (step < 0 && begin < end)) {
        return boost::irange(0, 0, step);
    } else {
        return boost::irange(begin, end, step);
    }
}

#endif

#ifdef USING_NAMESPACE
using namespace std;

#ifdef USING_BOOST
using namespace boost;
using namespace boost::adaptors;
#endif
#endif

#pragma endregion

/*** Template End ***/

#include<algorithm>
using namespace std;
using ll = long long;

#define N 100003

class SegmentTree {
  const ll inf = 1e18;
  int n, n0;
  ll max_v[4*N], smax_v[4*N];
  ll sum[4*N], max_c[4*N];

  void update_node_max(int k, ll x) {
    sum[k] += (x - max_v[k]) * max_c[k];
    max_v[k] = x;
  }

  void push(int k) {
    if(max_v[k] < max_v[2*k+1]) {
      update_node_max(2*k+1, max_v[k]);
    }
    if(max_v[k] < max_v[2*k+2]) {
      update_node_max(2*k+2, max_v[k]);
    }
  }

  void update(int k) {
    sum[k] = sum[2*k+1] + sum[2*k+2];

    if(max_v[2*k+1] < max_v[2*k+2]) {
      max_v[k] = max_v[2*k+2];
      max_c[k] = max_c[2*k+2];
      smax_v[k] = max(max_v[2*k+1], smax_v[2*k+2]);
    } else if(max_v[2*k+1] > max_v[2*k+2]) {
      max_v[k] = max_v[2*k+1];
      max_c[k] = max_c[2*k+1];
      smax_v[k] = max(smax_v[2*k+1], max_v[2*k+2]);
    } else {
      max_v[k] = max_v[2*k+1];
      max_c[k] = max_c[2*k+1] + max_c[2*k+2];
      smax_v[k] = max(smax_v[2*k+1], smax_v[2*k+2]);
    }
  }

  void _update_min(ll x, int a, int b, int k, int l, int r) {
    if(b <= l || r <= a || max_v[k] <= x) {
      return;
    }
    if(a <= l && r <= b && smax_v[k] < x) {
      update_node_max(k, x);
      return;
    }

    push(k);
    _update_min(x, a, b, 2*k+1, l, (l+r)/2);
    _update_min(x, a, b, 2*k+2, (l+r)/2, r);
    update(k);
  }

  ll _query_max(int a, int b, int k, int l, int r) {
    if(b <= l || r <= a) {
      return 0;
    }
    if(a <= l && r <= b) {
      return max_v[k];
    }
    push(k);
    ll lv = _query_max(a, b, 2*k+1, l, (l+r)/2);
    ll rv = _query_max(a, b, 2*k+2, (l+r)/2, r);
    return max(lv, rv);
  }

  ll _query_sum(int a, int b, int k, int l, int r) {
    if(b <= l || r <= a) {
      return 0;
    }
    if(a <= l && r <= b) {
      return sum[k];
    }
    push(k);
    ll lv = _query_sum(a, b, 2*k+1, l, (l+r)/2);
    ll rv = _query_sum(a, b, 2*k+2, (l+r)/2, r);
    return lv + rv;
  }

public:
  SegmentTree(int n) {
    SegmentTree(n, nullptr);
  }

  SegmentTree(int n, ll *a) : n(n) {
    n0 = 1;
    while(n0 < n) n0 <<= 1;

    for(int i=0; i<n; ++i) {
      max_v[n0-1+i] = sum[n0-1+i] = (a != nullptr ? a[i] : 0);
      smax_v[n0-1+i] = -inf;
      max_c[n0-1+i] = 1;
    }

    for(int i=n; i<n0; ++i) {
      max_v[n0-1+i] = smax_v[n0-1+i] = -inf;
      sum[n0-1+i] = max_c[n0-1+i] = 0;
    }
    for(int i=n0-2; i>=0; i--) update(i);
  }

  // range minimize query
  void update_min(int a, int b, ll x) {
    return _update_min(x, a, b, 0, 0, n0);
  }

  // range maximum query
  ll query_max(int a, int b) {
    return _query_max(a, b, 0, 0, n0);
  }

  // range sum query
  ll query_sum(int a, int b) {
    return _query_sum(a, b, 0, 0, n0);
  }
};

int main() {
    int n, m;
    in(n, m);

    vector<tuple<int, int, ll>> range;

    ll inf = 1LL << 60;

    
    for (int i = 0; i < m; i++) {
        int l, r;
        ll c;
        in(l, r, c);
        l--;
        range.emplace_back(l, r, c);
    }

    sort(range.begin(), range.end());

    vector<ll> a(n, inf);

    a[0] = 0;

    static SegmentTree seg(n, a.data());

    for (int i = 0; i < m; i++) {
        int l, r;
        ll c;

        tie(l, r, c) = range[i];

        ll x = seg.query_max(l, l + 1);

        if (x != inf) {
            seg.update_min(l, r, x + c);
        }
    }

    ll res = seg.query_max(n-1, n);

    if (res == inf) {
        out(-1);
    } else {
        out(res);
    }
}

Measured memory_bytes: 14832