using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Runtime.CompilerServices;
using System.Text;
using static Template;
using static System.Console;
using static System.Convert;
using static System.Math;
using Pi = Pair<int, int>;
using Number = System.Int64;

class Solver
{
    public void Solve()
    {
        int n, m;
        Input.Make(out n, out m);
        var dij = new Dijkstra(n);
        for (var i = 1; i < n; i++)
            dij.AddEdge(i, i - 1, 0);
        for(var i = 0; i < m; i++)
        {
            int l, r, c;
            Input.Make(out l, out r, out c);
            l--;r--;
            dij.AddEdge(l, r, c);
        }
        var res = dij.Execute();
        WriteLine(res[n-1] == long.MaxValue ? -1 : res[n-1]);
    }
}

class Dijkstra
{
    private int num;
    private List<Edge>[] edges;
    public Dijkstra(int num)
    { this.num = num; edges = Create(num, () => new List<Edge>()); }
    public void AddEdge(int from, int to, Number weight)
        => edges[from].Add(new Edge(to, weight));
    public Number[] Execute(int st = 0)
    {
        var dist = Create(num, () => Number.MaxValue);
        var pq = new PriorityQueue<Edge>();
        pq.Enqueue(new Edge(st, 0));
        dist[st] = 0;
        while (pq.Any())
        {
            var p = pq.Dequeue();
            if (p.cost > dist[p.to]) continue;
            foreach (var e in edges[p.to])
                if (chmin(ref dist[e.to], e.cost + p.cost))
                    pq.Enqueue(new Edge(e.to, dist[e.to]));
        }
        return dist;
    }

    struct Edge : IComparable<Edge>
    {
        public int to;
        public Number cost;
        public Edge(int to, Number cost)
        { this.to = to; this.cost = cost; }
        public int CompareTo(Edge e)
            => cost.CompareTo(e.cost);
    }
}

public class PriorityQueue<T>
{
    private List<T> item = new List<T>();
    private Comparison<T> cmp;
    public int Count { get { return item.Count; } }
    public T Peek { get { return item[0]; } }
    public PriorityQueue() { cmp = Comparer<T>.Default.Compare; }

    public PriorityQueue(Comparison<T> comparison) { cmp = comparison; }

    public PriorityQueue(IComparer<T> comparer) { cmp = comparer.Compare; }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private int Parent(int i)
        => (i - 1) >> 1;
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private int Left(int i)
        => (i << 1) + 1;
    public T Enqueue(T val)
    {
        int i = item.Count;
        item.Add(val);
        while (i > 0)
        {
            int p = Parent(i);
            if (cmp(item[p], val) <= 0)
                break;
            item[i] = item[p];
            i = p;
        }
        item[i] = val;
        return val;
    }
    public T Dequeue()
    {
        var ret = item[0];
        var p = 0;
        var x = item[item.Count - 1];
        while (Left(p) < item.Count - 1)
        {
            var l = Left(p);
            if (l < item.Count - 2 && cmp(item[l + 1], item[l]) < 0) l++;
            if (cmp(item[l], x) >= 0)
                break;
            item[p] = item[l];
            p = l;
        }
        item[p] = x;
        item.RemoveAt(item.Count - 1);
        return ret;
    }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public bool Any() => item.Count > 0;
}
#region Template
public class Template
{
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool chmin<T>(ref T num, T val) where T : IComparable<T>
    { if (num.CompareTo(val) == 1) { num = val; return true; } return false; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool chmax<T>(ref T num, T val) where T : IComparable<T>
    { if (num.CompareTo(val) == -1) { num = val; return true; } return false; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void swap<T>(ref T v1, ref T v2)
    { var t = v2; v2 = v1; v1 = t; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T[] Create<T>(int n, Func<T> f)
        => Enumerable.Repeat(0, n).Select(_ => f()).ToArray();
    public static void Fail() => Fail("No");
    public static void Fail<T>(T s) { Console.WriteLine(s); Console.Out.Close(); Environment.Exit(0); }
    static void Main(string[] args)
    {
        var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
        Console.SetOut(sw);
        var p = new Solver();
        for (var i = 1; i > 0; --i)
            p.Solve();
        Console.Out.Flush();
    }
}

public class Input
{
    public static string read => Console.ReadLine().Trim();
    public static int[] ar => read.Split(' ').Select(int.Parse).ToArray();
    public static int num => Convert.ToInt32(read);
    public static long[] arL => read.Split(' ').Select(long.Parse).ToArray();
    public static long numL => Convert.ToInt64(read);
    public static char[][] grid(int h)
        => Create(h, () => read.ToCharArray());
    public static int[] ar1D(int n)
        => Create(n, () => num);
    public static long[] arL1D(int n)
        => Create(n, () => numL);
    public static string[] strs(int n)
        => Create(n, () => read);
    public static int[][] ar2D(int n)
        => Create(n, () => ar);
    public static long[][] arL2D(int n)
        => Create(n, () => arL);
    public static List<T>[] edge<T>(int n)
        => Create(n, () => new List<T>());
    public static void Make<T1, T2>(out T1 v1, out T2 v2)
    {
        v1 = Next<T1>();
        v2 = Next<T2>();
    }
    public static void Make<T1, T2, T3>(out T1 v1, out T2 v2, out T3 v3)
    {
        Make(out v1, out v2);
        v3 = Next<T3>();
    }
    public static void Make<T1, T2, T3, T4>(out T1 v1, out T2 v2, out T3 v3, out T4 v4)
    {
        Make(out v1, out v2, out v3);
        v4 = Next<T4>();
    }
    public static void Make<T1, T2, T3, T4, T5>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5)
    {
        Make(out v1, out v2, out v3, out v4);
        v5 = Next<T5>();
    }
    public static void Make<T1, T2, T3, T4, T5, T6>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5, out T6 v6)
    {
        Make(out v1, out v2, out v3, out v4, out v5);
        v6 = Next<T6>();
    }
    static Input()
    {
        sc = new Queue<string>();
        dic = new Dictionary<Type, Func<string, object>>();
        dic[typeof(int)] = s => int.Parse(s);
        dic[typeof(long)] = s => long.Parse(s);
        dic[typeof(char)] = s => char.Parse(s);
        dic[typeof(double)] = s => double.Parse(s);
        dic[typeof(uint)] = s => uint.Parse(s);
        dic[typeof(ulong)] = s => ulong.Parse(s);
        dic[typeof(string)] = s => s;
    }
    private static Dictionary<Type, Func<string, object>> dic;
    private static Queue<string> sc;
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T Next<T>() { if (sc.Count == 0) foreach (var item in read.Split(' ')) sc.Enqueue(item); return (T)dic[typeof(T)](sc.Dequeue()); }
    public const int MOD = 1000000007;
}

public class Pair<T1, T2> : IComparable<Pair<T1, T2>>
{
    public T1 v1;
    public T2 v2;
    public Pair() { v1 = default(T1);v2 = default(T2); }
    public Pair(T1 v1, T2 v2)
    { this.v1 = v1; this.v2 = v2; }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public int CompareTo(Pair<T1, T2> p)
    {
        var c = Comparer<T1>.Default.Compare(v1, p.v1);
        if (c == 0)
            c = Comparer<T2>.Default.Compare(v2, p.v2);
        return c;
    }
    public override string ToString()
        => $"{v1.ToString()} {v2.ToString()}";
}

public class Pair<T1, T2, T3> : Pair<T1, T2>, IComparable<Pair<T1, T2, T3>>
{
    public T3 v3;
    public Pair() : base() { v3 = default(T3); }
    public Pair(T1 v1, T2 v2, T3 v3) : base(v1, v2)
    { this.v3 = v3; }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public int CompareTo(Pair<T1, T2, T3> p)
    {
        var c = base.CompareTo(p);
        if (c == 0)
            c = Comparer<T3>.Default.Compare(v3, p.v3);
        return c;
    }
    public override string ToString()
        => $"{base.ToString()} {v3.ToString()}";
}
#endregion


Measured memory_bytes: 38648