#include <bits/stdc++.h>
#define REP(i, n) for(int i = 0;i < n;i++)
#define VSORT(v) sort(v.begin(), v.end())
#define VRSORT(v) sort(v.rbegin(), v.rend())
#define ll long long
using namespace std;
typedef pair<int, int> P;
typedef pair<ll, ll> LP;
typedef pair<int, P> PP;
typedef pair<ll, LP> LPP;
//typedef vector<unsigned int>vec;
typedef vector<ll>vec;
typedef vector<vec> mat;
typedef vector<vector<int>> Graph;

const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
//const int INF = 1000000000;
const ll LINF = 1000000000000000000;//1e18
//const ll  MOD = 1000000007;
const ll MOD = 998244353;
const double PI = acos(-1.0);
const double EPS = 1e-10;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline void add(T &a, T b){a = ((a+b) % MOD + MOD) % MOD;};

template<class Monoid> struct SegTree {
    using Func = function<Monoid(Monoid, Monoid)>;
    const Func F;
    const Monoid UNITY;
    int SIZE_R;
    vector<Monoid> dat;
    
    SegTree(int n, const Func f, const Monoid &unity): F(f), UNITY(unity) { init(n); }
    void init(int n) {
        SIZE_R = 1;
        while (SIZE_R < n) SIZE_R *= 2;
        dat.assign(SIZE_R * 2, UNITY);
    }
    
    /* set, a is 0-indexed */
    void set(int a, const Monoid &v) { dat[a + SIZE_R] = v; }
    void build() {
        for (int k = SIZE_R - 1; k > 0; --k)
            dat[k] = F(dat[k*2], dat[k*2+1]);
    }
    
    /* update a, a is 0-indexed */
    void update(int a, const Monoid &v) {
        int k = a + SIZE_R;
        dat[k] = v;
        while (k >>= 1) dat[k] = F(dat[k*2], dat[k*2+1]);
    }
    
    /* get [a, b), a and b are 0-indexed */
    Monoid get(int a, int b) {
        Monoid vleft = UNITY, vright = UNITY;
        for (int left = a + SIZE_R, right = b + SIZE_R; left < right; left >>= 1, right >>= 1) {
            if (left & 1) vleft = F(vleft, dat[left++]);
            if (right & 1) vright = F(dat[--right], vright);
        }
        return F(vleft, vright);
    }
    inline Monoid operator [] (int a) { return dat[a + SIZE_R]; }

    /* debug */
    void print() {
        for (int i = 0; i < SIZE_R; ++i) {
            cout << (*this)[i];
            if (i != SIZE_R-1) cout << ",";
        }
        cout << endl;
    }
};

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N, M;
    cin >> N >> M;
    vector<tuple<ll, ll, ll>> LRC(M);
    REP(i,M){
        ll L, R, C;
        cin >> L >> R >> C;
        L--, R--;
        LRC[i] = make_tuple(L, R, C);
    }
    sort(LRC.begin(), LRC.end());
    vector<ll> dist(N, LINF);
    dist[0] = 0;
    SegTree<long long> seg(N, [](long long a, long long b) {
            return min(a,b);
        },
        LINF);
    for (int i = 0; i < N; ++i) {
        seg.set(i, dist[i]);
    }
    seg.build();
    REP(i,M){
        ll L, R, C;
        tie(L, R, C) = LRC[i];
        ll mi = seg.get(L, R+1);
        if(mi == LINF) continue;
        chmin(dist[R], mi + C);
        seg.update(R, dist[R]);
    }
    if(dist[N-1] == LINF) cout << -1 << endl;
    else cout << dist[N-1] << endl;

}

Measured memory_bytes: 5376