using System;
using System.Collections.Generic;
using static Assistant.Input;
using static Assistant.Debug;
using System.Linq;
using Assistant;

namespace 第二回全国統一プログラミング王決定戦予選D
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = RInt;
            var M = RInt;

            Dijkstra dijkstra = new Dijkstra(N);
            for (int i = 0; i < M; i++)
            {
                var l = RInt - 1;
                var r = RInt - 1;
                var c = RInt;
                dijkstra.Add(l, r, c);
            }
            for (int i = 1; i < N; i++)
            {
                dijkstra.Add(i, i - 1, 0);
            }

            var distance = dijkstra.Start(0);

            var ans = distance.Last();
            if (ans == long.MaxValue / 2) ans = -1;
            Console.WriteLine(ans);
        }

        struct Edge
        {
            public int to;
            public long cost;
        }

    }

    class Dijkstra
    {
        public long INF = long.MaxValue / 2;
        public int N { get; }               // 頂点の数
        private List<Edge>[] _graph;        // グラフの辺のデータ

        public Dijkstra(int n)
        {
            N = n;
            _graph = new List<Edge>[n];
            for (int i = 0; i < n; i++) _graph[i] = new List<Edge>();
        }

        public void Add(int a, int b, long cost = 1)
                => _graph[a].Add(new Edge(b, cost));

        public long[] Start(int start)
        {
            // コストをスタート頂点以外を無限大に
            var cost = new long[N];
            for (int i = 0; i < N; i++) cost[i] = INF;
            cost[start] = 0;

            // 未確定の頂点を格納する優先度付きキュー(コストが小さいほど優先度が高い)
            var q = new PriorityQueue<Vertex>(Comparer<Vertex>.Create((a, b) => b.CompareTo(a)), N * 10);
            q.Push(new Vertex(start, 0));

            while (q.Count() > 0)
            {
                var v = q.Pop();

                // 記録されているコストと異なる(コストがより大きい)場合は無視
                if (v.cost != cost[v.index]) continue;

                foreach (var e in _graph[v.index])
                {
                    // 既に記録されているコストより小さければコストを更新
                    if (cost[e.to] > v.cost + e.cost)
                    {
                        cost[e.to] = v.cost + e.cost;
                        q.Push(new Vertex(e.to, cost[e.to]));
                    }
                }
            }

            // 確定したコストを返す
            return cost;
        }

        public struct Edge
        {
            public int to;                      // 接続先の頂点
            public long cost;                   // 辺のコスト

            public Edge(int to, long cost)
            {
                this.to = to;
                this.cost = cost;
            }
        }

        public struct Vertex : IComparable<Vertex>
        {
            public int index;                   // 頂点の番号
            public long cost;                   // 記録したコスト

            public Vertex(int index, long cost)
            {
                this.index = index;
                this.cost = cost;
            }

            public int CompareTo(Vertex other)
                => cost.CompareTo(other.cost);
        }
    }

    /// <summary>
    /// PriorityQue
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public class PriorityQueue<T> where T : IComparable<T>
    {
        private IComparer<T> _comparer = null;
        private int _type = 0;

        private T[] _heap;
        private int _sz = 0;

        private int _count = 0;

        /// <summary>
        /// Priority Queue with custom comparer
        /// </summary>
        public PriorityQueue(IComparer<T> comparer, int capacity = 128)
        {
            _heap = new T[capacity];
            _comparer = comparer;
        }

        /// <summary>
        /// Priority queue
        /// </summary>
        /// <param name="type">0: asc, 1:desc</param>
        public PriorityQueue(int type = 0)
        {
            _heap = new T[128];
            _type = type;
        }

        private int Compare(T x, T y)
        {
            if (_comparer != null) return _comparer.Compare(x, y);
            return _type == 0 ? x.CompareTo(y) : y.CompareTo(x);
        }

        public void Push(T x)
        {
            _count++;
            if (_count > _heap.Length)
            {
                var newheap = new T[_heap.Length * 2];
                for (int n = 0; n < _heap.Length; n++) newheap[n] = _heap[n];
                _heap = newheap;
            }

            //node number
            var i = _sz++;

            while (i > 0)
            {
                //parent node number
                var p = (i - 1) / 2;

                if (Compare(_heap[p], x) <= 0) break;

                _heap[i] = _heap[p];
                i = p;
            }

            _heap[i] = x;
        }

        public T Pop()
        {
            _count--;

            T ret = _heap[0];
            T x = _heap[--_sz];

            int i = 0;
            while (i * 2 + 1 < _sz)
            {
                //children
                int a = i * 2 + 1;
                int b = i * 2 + 2;

                if (b < _sz && Compare(_heap[b], _heap[a]) < 0) a = b;

                if (Compare(_heap[a], x) >= 0) break;

                _heap[i] = _heap[a];
                i = a;
            }

            _heap[i] = x;

            return ret;
        }

        public int Count()
        {
            return _count;
        }

        public T Peek()
        {
            return _heap[0];
        }

        public bool Contains(T x)
        {
            for (int i = 0; i < _sz; i++) if (x.Equals(_heap[i])) return true;
            return false;
        }

        public void Clear()
        {
            while (this.Count() > 0) this.Pop();
        }

        public IEnumerator<T> GetEnumerator()
        {
            var ret = new List<T>();

            while (this.Count() > 0)
            {
                ret.Add(this.Pop());
            }

            foreach (var r in ret)
            {
                this.Push(r);
                yield return r;
            }
        }

        public T[] ToArray()
        {
            T[] array = new T[_sz];
            int i = 0;

            foreach (var r in this)
            {
                array[i++] = r;
            }

            return array;
        }
    }

}

namespace Assistant
{
    static class Input
    {
        static List<string> line = new List<string>();
        static int index = 0;
        static String RNext()
        {
            if (line.Count <= index) line.AddRange(Console.ReadLine().Split());
            return line[index++];
        }
        public static int RInt => int.Parse(RNext());
        public static long RLong => long.Parse(RNext());
        public static int[] RInts => Console.ReadLine().Split().Select(int.Parse).ToArray();
        public static long[] RLongs => Console.ReadLine().Split().Select(long.Parse).ToArray();
        public static string RString => RNext();
        //以下未テスト
        public static int[] RIntsC(int c) => Enumerable.Repeat(0, c).Select(x => int.Parse(RNext())).ToArray();
        public static long[] RLongsC(int c) => Enumerable.Repeat(0, c).Select(x => long.Parse(RNext())).ToArray();
        public static char[][] RMap(int h) => Enumerable.Repeat(0, h).Select(x => Console.ReadLine().ToCharArray()).ToArray();
    }

    public struct Mlong
    {
        long _v;
        const long mod = 1000000007;
        public Mlong(long n = 0) : this() { _v = n >= mod ? n % mod : n; }

        public static implicit operator Mlong(long _x) => new Mlong(_x);
        public static implicit operator long(Mlong _x) => _x._v;
        public static Mlong operator +(Mlong m1, Mlong m2) { long m = m1._v + m2._v; return m >= mod ? m - mod : m; }
        public static Mlong operator -(Mlong m1, Mlong m2) { long m = m1._v - m2._v; return m >= 0 ? m : m + mod; }
        public static Mlong operator *(Mlong m1, Mlong m2) => m1._v * m2._v % mod;
        public static Mlong operator /(Mlong m1, Mlong m2) => m1._v * ModPow(m2._v, mod - 2) % mod;

        public static long ModPow(long a, long n)
        {
            if (n == 0) return 1;
            else if (n % 2 == 1) return a * ModPow(a, n - 1) % mod;
            else return ModPow(a * a % mod, n / 2);
        }

        static Mlong[] fac, finv, inv;
        public static void nCkInit(int max)
        {
            fac = new Mlong[max]; finv = new Mlong[max]; inv = new Mlong[max];
            fac[0] = fac[1] = 1;
            finv[0] = finv[1] = 1;
            inv[1] = 1;
            for (int i = 2; i < max; i++)
            {
                fac[i] = fac[i - 1] * i;
                inv[i] = mod - inv[mod % i] * (mod / i);
                finv[i] = finv[i - 1] * inv[i];
            }
        }

        public static Mlong nCk(int n, int k)
        {
            if (n < k) return 0;
            if (n < 0 || k < 0) return 0;
            return fac[n] * finv[k] * finv[n - k];
        }
    }

    static class Debug
    {
        static public void Draw2D<T>(T[,] map, int mode = 1)
        {
#if DEBUG
            int W = map.GetLength(0);
            int H = map.GetLength(1);

            string[,] map2 = new string[W + 1, H + 1];
            for (int i = 0; i < W + 1; i++)
            {
                for (int j = 0; j < H + 1; j++)
                {
                    if (i == 0 && j == 0) map2[i, j] = 0.ToString();
                    else if (i == 0) map2[i, j] = (j - 1).ToString();
                    else if (j == 0) map2[i, j] = (i - 1).ToString();
                    else map2[i, j] = map[i - 1, j - 1].ToString();
                }
            }

            for (int i = 0; i < W + 1; i++)
            {
                for (int j = 0; j < H + 1; j++)
                {
                    if (mode == 0) Console.Write(map2[i, j].Last());
                    if (mode == 1) Console.Write(map2[i, j] + " ");
                }
                Console.WriteLine();
            }
            Console.WriteLine();
#endif
        }

        public static void Draw1D<T>(T[] array, int mode = 0)
        {
#if DEBUG
            Console.WriteLine(string.Join(" ", array));
#endif
        }
    }
}
