#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < int(n); i++)
#define rep1(i,n) for (int i = 1; i <= int(n); i++)
#define repR(i,n) for (int i = int(n)-1; i >= 0; i--)
#define rep1R(i,n) for (int i = int(n); i >= 1; i--)
#define loop(i,a,B) for (int i = a; i B; i++)
#define loopR(i,a,B) for (int i = a; i B; i--)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define eb emplace_back
#define mp make_pair
#define fst first
#define snd second
#ifdef LOCAL
#define dump(...) cerr << "[" << __LINE__ << ":" << __FUNCTION__ << "] ", my_dmp(#__VA_ARGS__, __VA_ARGS__)
#define say(x) cerr << "[" << __LINE__ << ":" << __FUNCTION__ << "] " << x << endl;
void my_dmp(const char*) { cerr << endl; }
template <class T, class... U> void my_dmp(const char *s, T const& x, U const& ...y) { const char *o = "({[", *e = "]})"; for (int i = 0; *s != '\0'; cerr << *s++) { if (count(o,o+3,*s)) i++; if (count(e,e+3,*s)) i--; if (!i && *s == ',') break; } cerr << " = " << x; if (*s == ',') cerr << ", ", s++; my_dmp(s, y...); }
#else
#define dump(...)
#define say(x)
#endif
using ll = long long;
using ld = long double;
#define int ll
#define double ld
template <class T> using pque_max = priority_queue<T>;
template <class T> using pque_min = priority_queue<T, vector<T>, greater<T> >;
template <class T, class = typename T::iterator, class = typename enable_if<!is_same<T, string>::value>::type>
ostream& operator<<(ostream& os, T const& v) { os << "{"; for (auto const& x : v) os << " " << x; return os << " }"; }
template <class T> istream& operator>>(istream& is, vector<T>& v) { for (auto& x : v) is >> x; return is; }
template <class T, class S> ostream& operator<<(ostream& os, pair<T,S> const& p) { return os << "(" << p.first << ", " << p.second << ")"; }
template <class T, class S> istream& operator>>(istream& is, pair<T,S>& p) { return is >> p.first >> p.second; }
template <size_t i, class T> typename enable_if<i >= tuple_size<T>::value>::type output_tuple(ostream&, T const&) { }
template <size_t i = 0, class T> typename enable_if<i < tuple_size<T>::value>::type
output_tuple(ostream& os, T const& t) { os << (i ? " " : "") << get<i>(t); output_tuple<i+1,T>(os,t); }
template <class... T> ostream& operator<<(ostream& os, tuple<T...> const& t) { return output_tuple(os,t), os; }
struct my_Init { my_Init() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } my_init;
template <class T, size_t d> struct vec_impl {
    using type = vector<typename vec_impl<T,d-1>::type>;
    template <class... U> static type make_v(size_t n, U&&... x) { return type(n, vec_impl<T,d-1>::make_v(forward<U>(x)...)); }
};
template <class T> struct vec_impl<T,0> { using type = T; static type make_v(T const& x = {}) { return x; } };
template <class T, size_t d> using vec = typename vec_impl<T,d>::type;
template <class T, size_t d, class... Args> auto make_v(Args&&... args) { return vec_impl<T,d>::make_v(forward<Args>(args)...); }
template <class T> void quit(T const& x) { cout << x << endl; exit(0); }
template <class T> constexpr bool chmin(T& x, T const& y) { if (x > y) { x = y; return true; } return false; }
template <class T> constexpr bool chmax(T& x, T const& y) { if (x < y) { x = y; return true; } return false; }
template <class It> constexpr auto sumof(It b, It e) { return accumulate(b,e,typename iterator_traits<It>::value_type{}); }
const ll INF = (1LL<<62)-1; // ~ 4.6e18
////////////////////////////////
//
template <class Handler>
struct Segtree : Handler {
    using Value = typename Handler::Value;
    using Handler::unit; // () -> Value
    using Handler::merge; // (Value,Value) -> Value

    vector<Value> v; // use v[1..2*cap-1]
    int cap; // capacity: power of 2
    int n; // original size

    Segtree() {}
    Segtree(int n) { init(n); }
    template <class It> Segtree(It first, It last) { init(first, last); }
    void init(int n) {
        assert(n >= 0); this->n = n;
        for (cap = 1; cap < n; cap <<= 1) ;;
		v.assign(2*cap, unit());
    }
    template <class It> void init(It first, It last) {
        init(distance(first,last));
        copy(first, last, next(v.begin(), cap));
        for (int i = cap-1; i >= 1; i--) v[i] = merge(v[2*i],v[2*i+1]);
    }
    int size() const { return n; }
    void set(int i, Value const& x) {
        assert(0 <= i && i < size());
        i += cap; v[i] = x;
        while (i > 1) i >>= 1, v[i] = merge(v[2*i],v[2*i+1]);
    }
    Value operator[](int i) const { return get(i); }
    Value get(int i) const {
        assert(0 <= i && i < size());
        return v[cap + i];
    }
    // [l,r)
    Value get(int l, int r) const {
        assert(0 <= l && l <= r && r <= size());
        Value x = unit(), y = unit();
        for (l += cap, r += cap; l < r; l >>= 1, r >>= 1) {
            if (l&1) x = merge(x, v[l++]);
            if (r&1) y = merge(v[--r], y);
        }
        return merge(x,y);
    }
    vector<Value> dat() const {
        vector<Value> ret(size());
        for (int i = 0; i < size(); i++) ret[i] = get(i);
        return ret;
    }
};

struct RangeMin {
    using Value = int;
    constexpr static Value unit() { return INF; }
    constexpr static Value merge(Value x, Value y) { return min(x,y); }
};

#define cost fst
#define from snd
int32_t main() {
    int n,m; cin >> n >> m;
    vec<pair<int,int>,2> g(n);
    rep (i,m) {
        int l,r,c; cin >> l >> r >> c; l--,r--;
        g[r].eb(c,l);
    }
    Segtree<RangeMin> dp(n);
    dp.set(0,0);
    rep (i,n-1) for (auto e : g[i+1]) {
        dp.set(i+1, min(dp[i+1], dp.get(e.from,i+1) + e.cost));
    }
    int ans = dp[n-1];
    if (ans == INF) ans = -1;
    cout << ans << endl;

    //
    dump(dp.dat());
}


Measured memory_bytes: 7680