//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <chrono>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>

using namespace std;

// typedef
//------------------------------------------
typedef long long LL;
typedef vector<int> VI;
typedef vector<bool> VB;
typedef vector<char> VC;
typedef vector<double> VD;
typedef vector<string> VS;
typedef vector<LL> VLL;
typedef vector<VI> VVI;
typedef vector<VB> VVB;
typedef vector<VS> VVS;
typedef vector<VLL> VVLL;
typedef vector<VVI> VVVI;
typedef vector<VVLL> VVVLL;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef pair<int, string> PIS;
typedef pair<string, int> PSI;
typedef pair<string, string> PSS;
typedef vector<PII> VPII;
typedef vector<PLL> VPLL;
typedef vector<VPII> VVPII;
typedef vector<VPLL> VVPLL;
typedef vector<VS> VVS;
typedef map<int, int> MII;
typedef map<LL, LL> MLL;
typedef map<string, int> MSI;
typedef map<int, string> MIS;

// to-method
//------------------------------------------
inline int toInt(string s) {
    int v;
    istringstream sin(s);
    sin >> v;
    return v;
}

inline LL toLongLong(string s) {
    LL v;
    istringstream sin(s);
    sin >> v;
    return v;
}

template<class T>
inline string toString(T x) {
    ostringstream sout;
    sout << x;
    return sout.str();
}

inline VC toVC(string s) {
    VC data(s.begin(), s.end());
    return data;
}



// container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define SZ(a) int((a).size())
#define EACH(i, arr) for(typeof((arr).begin()) i=(arr).begin(); i!=(arr).end(); ++i)
#define EXIST(str, e) ((str).find(e)!=(str).end())
#define COUNT(arr, v) count((arr).begin(), (arr).end(), v)
#define SEARCH(v, w) search((v).begin(), (v).end(), (w).begin(), (w).end())
#define SORT(c) sort((c).begin(),(c).end())
#define RSORT(c) sort((c).rbegin(),(c).rend())
#define REVERSE(c) reverse((c).begin(), (c).end())
#define ROTATE_LEFT(arr, c) rotate((arr).begin(), (arr).begin()+(c), (arr).end())
#define ROTATE_RIGHT(arr, c) rotate((arr).rbegin(), (arr).rbegin() + (c), (arr).rend())
#define SUMI(arr) accumulate((arr).begin(), (arr).end(), 0)
#define SUMD(arr) accumulate((arr).begin(), (arr).end(), 0.)
#define SUMLL(arr) accumulate((arr).begin(), (arr).end(), 0LL)
#define SUMS(arr) accumulate((arr).begin(), (arr).end(), string())
#define MULD(arr) accumulate((arr).begin(), (arr).end(), 1., multiplies<double>())
#define MULLL(arr) accumulate((arr).begin(), (arr).end(), 1LL, multiplies<long long>())
#define UB(arr, n) upper_bound((arr).begin(), (arr).end(), n)
#define LB(arr, n) lower_bound((arr).begin(), (arr).end(), n)
#define OF_ALL(arr, func) all_of((arr).begin(), (arr).end(), (func))
#define OF_NONE(arr, func) none_of((arr).begin(), (arr).end(), (func))
#define OF_ANY(arr, func) any_of((arr).begin(), (arr).end(), (func))
#define PB push_back
#define MP make_pair
#define ft first
#define sd second



// input output
//------------------------------------------
#define GL(s) getline(cin, (s))
#define INIT() std::ios::sync_with_stdio(false);std::cin.tie(0)
#define OUT(d) std::cout<<(d)
#define OUT_L(d) std::cout<<(d)<<endl
#define FOUT(n, data) std::cout<<std::fixed<<std::setprecision(n)<<(data)
#define FOUT_L(n, data) std::cout<<std::fixed<<std::setprecision(n)<<(data)<<"\n"
#define EL() printf("\n")
#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v) {std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}
#define Yes() printf("Yes\n")
#define No() printf("No\n")
#define YES() printf("YES\n")
#define NO() printf("NO\n")
#define Yay() printf("Yay!\n")
#define Nnn() printf(":(\n")


template<typename T1, typename T2>
istream &operator>>(istream &in, pair<T1, T2> &p) {
    in >> p.first >> p.second;
    return in;
}

template<typename T>
istream &operator>>(istream &in, vector<T> &v) {
    for (auto &x: v)
        in >> x;
    return in;
}


template<typename T1, typename T2>
ostream &operator<<(ostream &out, const std::pair<T1, T2> &p) {
    out << "[" << p.first << ", " << p.second << "]" << "\n";
    return out;
}


//repetition
//------------------------------------------
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-1;i>=(a);--i)
#define REP(i, n)  FOR(i,0,n)
#define RREP(i, n) for(int i = n-1;i >= 0;i--)
#define FORLL(i, a, b) for(LL i=LL(a);i<LL(b);++i)
#define RFORLL(i, a, b) for(LL i=LL(b)-1;i>=LL(a);--i)
#define REPLL(i, n) for(LL i=0;i<LL(n);++i)
#define RREPLL(i, n) for(LL i=LL(n)-1;i>=0;--i)
#define FOREACH(x, arr) for(auto &(x) : (arr))
#define FORITER(x, arr) for(auto (x) = (arr).begin(); (x) != (arr).end(); ++(x))

template<class T>
void chmin(T &a, const T b) { a = min(a, b); }
template<class T>
void chmax(T &a, const T b) { a = max(a, b); }

//------------------------------------------
//------------------------------------------

template<class T>
struct Edge {
    int from, to;
    T cost;

    Edge(int from, int to, T cost) : from(from), to(to), cost(cost) {}

    explicit operator int() const { return to; }
};

template<class T>
using Edges = vector<Edge<T>>;

template<class T>
using WeightedGraph = vector<Edges<T>>;

using UnWeightedGraph = vector<vector<int>>;

template<class T>
using DistMatrix = vector<vector<T>>;

struct GraphAdapter {
    template<class T>
    static UnWeightedGraph to_unweighted_graph(const WeightedGraph<T> &origin) {
        int V = origin.size();
        UnWeightedGraph graph(V);
        for (int i = 0; i < V; i++) for (auto &e: origin[i]) graph[i].push_back((int) e);
        return graph;
    }

    static WeightedGraph<int> to_weighted_graph(const UnWeightedGraph &origin) {
        int V = origin.size();
        WeightedGraph<int> graph(V);
        for (int i = 0; i < V; i++) for (auto to: origin[i]) graph[i].push_back({i, to, 1});
        return graph;
    }

    template<class T>
    static DistMatrix<T> to_dist_matrix(const WeightedGraph<T> &origin, T INF) {
        int V = origin.size();
        DistMatrix<T> matrix(V, vector<T>(V, INF));
        for (int i = 0; i < V; i++) for (auto &e:origin[i]) matrix[i][e.to] = e.cost;
        for (int i = 0; i < V; i++) matrix[i][i] = 0;
        return matrix;
    }
};

template<typename T>
class Dijkstra {
    using Pr = pair<T, int>;
private:
    int V;
    const WeightedGraph<T> &graph;
    const T INF = numeric_limits<T>::max();
    vector<T> dist;
    vector<int> prev;

public:

    Dijkstra(const WeightedGraph<T> &graph, int s) : graph(graph) {
        V = graph.size();
        dijkstra(s);
    }

    void dijkstra(int s) {
        priority_queue<Pr, vector<Pr>, greater<>> Q;
        dist = vector<T>(V, INF);
        prev = vector<int>(V, -1);
        Q.push(make_pair(0, s));
        dist[s] = 0;
        while (!Q.empty()) {
            auto p = Q.top();
            Q.pop();
            T now = p.first;
            int v = p.second;
            if (dist[v] < now)continue;
            for (auto e: graph[v]) {
                int to = e.to;
                T cost = e.cost;
                if (dist[to] > dist[v] + cost) {
                    dist[to] = dist[v] + cost;
                    prev[to] = v;
                    Q.push(make_pair(dist[to], to));
                }
            }
        }
    }

    vector<T> get_dist() const {
        return dist;
    }

    vector<int> get_prev() const {
        return prev;
    }

};

int main() {

    int N, M;
    cin >> N >> M;

    WeightedGraph<LL> graph(N);
    for (int i = 0; i < N - 1; i++) graph[i + 1].push_back({i + 1, i, 0});

    for (int i = 0; i < M; i++) {
        int l, r;
        cin >> l >> r;
        l--, r--;
        LL c;
        cin >> c;
        graph[l].push_back({l, r, c});
    }

    Dijkstra<LL> dijkstra(graph, 0);

    auto dist = dijkstra.get_dist();

    if (dist[N - 1] == numeric_limits<LL>::max()) cout << -1 << endl;
    else cout << dist[N - 1] << endl;

    return 0;
}






































Measured memory_bytes: 10488