#include <bits/stdc++.h>
using namespace std;
/*#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
template<typename T> using gpp_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T, typename L> using gpp_map = tree<T, L, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T> using gpp_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;*/
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define FOR(i, begin, end) for(int i=(begin);i<(end);i++)
#define REP(i, n) FOR(i,0,n)
#define IFOR(i, begin, end) for(int i=(end)-1;i>=(begin);i--)
#define IREP(i, n) IFOR(i,0,n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(),v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x) distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x) distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
#define bit(n) (1LL<<(n))
#define bit_exist(x, n) ((x >> n) & 1)
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v) cout << #v << "=" << endl; REP(i_debug, v.size()){ cout << v[i_debug] << ","; } cout << endl;
#define mdebug(m) cout << #m << "=" << endl; REP(i_debug, m.size()){ REP(j_debug, m[i_debug].size()){ cout << m[i_debug][j_debug] << ","; } cout << endl;}
#define pb push_back
#define f first
#define s second
#define int long long
#define INF 1000000000000000000
template<typename T> istream &operator>>(istream &is, vector<T> &v){ for (auto &x : v) is >> x; return is; }
template<typename T> ostream &operator<<(ostream &os, vector<T> &v){ for(int i = 0; i < v.size(); i++) { cout << v[i]; if(i != v.size() - 1) cout << endl; }; return os; }
template<typename T> void Out(T x) { cout << x << endl; }
template<typename T1, typename T2> void Ans(bool f, T1 y, T2 n) { if(f) Out(y); else Out(n); }

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using PiP = pair<int, Pii>;
using PPi = pair<Pii, int>;
using bools = vector<bool>;
using pairs = vector<Pii>;

//int dx[4] = {1,0,-1,0};
//int dy[4] = {0,1,0,-1};
//char d[4] = {'D','R','U','L'};

const int mod = 1000000007;
//const int mod = 998244353;
//#define Add(x, y) x = (x + (y)) % mod
//#define Mult(x, y) x = (x * (y)) % mod

struct LazySegmentTree
{
    //A: data, B: 作用素
    using T_A = int;
    using T_B = int;
    T_A id_A = INF;
    T_B id_B = INF;
    int N;
    vector<T_A> dat;
    vector<T_B> ope;

    T_A F_AA(T_A &u, T_A &v){
        return min(u, v);
    }

    T_A F_AB(T_A &u, T_B &v, int sz){
        return min(u, v);
    }

    T_B F_BB(T_B &u, T_B &v){
        return min(u, v);
    }

    LazySegmentTree(int n){
        N = 1;
        while(n > N) N = N << 1;
        dat = vector<T_A>(2 * N - 1, id_A);
        ope = vector<T_B>(2 * N - 1, id_B);
    }

    LazySegmentTree(int n, vector<T_A> &v){
        N = 1;
        while(n > N) N = N << 1;
        dat = vector<T_A>(2 * N - 1, id_A);
        ope = vector<T_B>(2 * N - 1, id_B);
        REP(i, n) dat[i + N - 1] = v[i];
        IREP(i, N - 1) dat[i] = F_AA(dat[i * 2 + 1], dat[i * 2 + 2]); 
    }

    //遅延評価
    void eval(int k, int l, int r){
        if(ope[k] != id_B){
            dat[k] = F_AB(dat[k], ope[k], r - l);
            if(r - l > 1){
                ope[k * 2 + 1] = F_BB(ope[k * 2 + 1], ope[k]);
                ope[k * 2 + 2] = F_BB(ope[k * 2 + 2], ope[k]);
            }
            ope[k] = id_B;
        }
    }

    void update(int a, int b, T_B x, int k, int l, int r){

        eval(k, l, r);
        if(b <= l || r <= a) return;

        if(a <= l && r <= b){
            ope[k] = F_BB(ope[k], x);
            eval(k, l, r);
        }else{
            update(a, b, x, k * 2 + 1, l, (l + r) / 2);
            update(a, b, x, k * 2 + 2, (l + r) / 2, r);
            dat[k] = F_AA(dat[k * 2 + 1], dat[k * 2 + 2]);
        }

    }

    T_A get(int a, int b, int k, int l, int r){
        if(r <= a || b <= l) return id_A;

        eval(k, l, r);
        if(a <= l && r <= b) return dat[k];
        else{
            T_A vl = get(a, b, k * 2 + 1, l, (l + r) / 2);
            T_A vr = get(a, b, k * 2 + 2, (l + r) / 2, r);
            return F_AA(vl, vr);
        }
    }

    //[a,b)の区間更新
    void update(int a, int b, T_B x){ update(a, b, x, 0, 0, N); }

    //[a, b)の演算結果
    T_A get(int a, int b){ return get(a, b, 0, 0, N); }

    T_A val(int k){ return get(k, k + 1); }
};

signed main(){

    int N, M; cin >> N >> M;
    vector<PPi> p(M);
    REP(i, M){
        int L, R, C; cin >> L >> R >> C;
        L--; R--;
        p[i] = PPi(Pii(L, R), C);
    }
    Sort(p);
    
    LazySegmentTree ST(N);
    ST.update(0, 1, 0);

    REP(i, M){
        int L = p[i].f.f, R = p[i].f.s, C = p[i].s;
        int m = ST.get(L, R + 1);
        ST.update(L, R + 1, m + C);
    }
    int ans = ST.val(N - 1);
    Ans(ans < INF, ans, -1);

    return 0;
}


Measured memory_bytes: 6656