object Main {
  import java.io.{BufferedReader, InputStream, InputStreamReader}
  import java.util.StringTokenizer
  import scala.reflect.ClassTag

  def main(args: Array[String]): Unit = {
    val out = new java.io.PrintWriter(System.out)
    new Main(out, new InputReader(System.in)).solve()
    out.flush()
  }

  private[this] val oj = System.getenv("ATCODER_DEBUG") == null
  @inline private final def DEBUG(f: => Unit): Unit = {
    if (!oj){ f }
  }
  @inline private final def debug(as: Array[Boolean]): Unit = if (!oj){ debug(as.map(x => if(x) "1" else "0").mkString) }
  @inline def debug(as: Array[Int]): Unit = if (!oj){ debug(as.mkString(" ")) }
  @inline private final def debug(as: Array[Long]): Unit =if (!oj){ debug(as.mkString(" ")) }
  @inline private final def debugDim(m: Array[Array[Int]]): Unit = if (!oj){
    REP(m.length) { i =>
      debug(m(i))
    }
  }
  @inline private final def debugDimFlip(m: Array[Array[Long]]): Unit = if (!oj){
    REP(m(0).length) { j =>
      REP(m.length) { i =>
        System.err.print(m(i)(j))
        System.err.print(" ")
      }
      System.err.println()
    }
  }
  @inline def debug(s: => String): Unit = DEBUG {
    System.err.println(s)
  }
  @inline private final def debugL(num: => Long): Unit = DEBUG {
    System.err.println(num)
  }
  private def isDebug[A](debug: => A, online: => A): A = {
    if (oj) online else debug
  }

  class InputReader(val stream: InputStream) {
    private[this] val reader = new BufferedReader(new InputStreamReader(stream), 32768)
    private[this] var tokenizer: StringTokenizer = _

    @inline private[this] final def next(): String = {
      while (tokenizer == null || !tokenizer.hasMoreTokens)
        tokenizer = new StringTokenizer(reader.readLine)
      tokenizer.nextToken
    }

    def nextInt(): Int = Integer.parseInt(next())
    def nextLong(): Long = java.lang.Long.parseLong(next())
    def nextChar(): Char = next().charAt(0)

    def ni(): Int = nextInt()
    def nl(): Long = nextLong()
    def nc(): Char = nextChar()
    def ns(): String = next()
    def ns(n: Int): Array[Char] = ns().toCharArray
    def na(n: Int, offset: Int = 0): Array[Int] = map(n)(_ => ni() + offset)
    def na2(n: Int, offset: Int = 0): (Array[Int], Array[Int]) = {
      val A1, A2 = Array.ofDim[Int](n)
      REP(n) { i =>
        A1(i) = ni() + offset
        A2(i) = ni() + offset
      }
      (A1, A2)
    }
    def nm(n: Int, m: Int): Array[Array[Int]] = {
      val A = Array.ofDim[Int](n, m)
      REP(n) { i =>
        REP(m) { j =>
          A(i)(j) = ni()
        }
      }
      A
    }
    def nal(n: Int): Array[Long] = map(n)(_ => nl())
    def nm_c(n: Int, m: Int): Array[Array[Char]] = map(n) (_ => ns(m))
  }

  def REP(n: Int, offset: Int = 0)(f: Int => Unit): Unit = {
    var i = offset
    val N = n + offset
    while(i < N) { f(i); i += 1 }
  }
  def REP_r(n: Int, offset: Int = 0)(f: Int => Unit): Unit = {
    var i = n - 1 + offset
    while(i >= offset) { f(i); i -= 1 }
  }
  def TO(from: Int, to: Int)(f: Int => Unit): Unit = {
    REP(to - from + 1, from)(f)
  }
  def map[@specialized A: ClassTag](n: Int, offset: Int = 0)(f: Int => A): Array[A] = {
    val res = Array.ofDim[A](n)
    REP(n)(i => res(i) = f(i + offset))
    res
  }

  def sumL(as: Array[Int]): Long = {
    var s = 0L
    REP(as.length)(i => s += as(i))
    s
  }
  def cumSum(as: Array[Int]): Array[Long] = {
    val cum = Array.ofDim[Long](as.length + 1)
    REP(as.length) { i =>
      cum(i + 1) = cum(i) + as(i)
    }
    cum
  }
}

object Workspace {
  import Main._

  case class Entry(l: Int, r: Int, c: Int)

  class SegmentTree(n: Int, zero: Long)(f: (Long, Long) => Long) {
   private val N = {
     val a = Integer.highestOneBit(n)
     if (a == n) a else a << 1
   }
   private val dat: Array[Long] = if (zero != 0){
     Array.fill(2 * N)(zero)
   } else {
     Array.ofDim(2 * N)
   }

   def update(i: Int, a: Long): Unit = {
     assert(i < n)
     var ix = i + N
     dat(ix) = a
     while(ix > 1) {
       dat(ix >> 1) = f(dat(ix), dat(ix ^ 1))
       ix >>= 1
     }
   }

   /**
     * [a, b)
     */
   def query(a: Int, b: Int): Long = {
     assert(a < n && b <= n)

     var res: Long = zero
     var left = a + N
     var right = b - 1 + N

     while(left <= right) {
       if ((left & 1) == 1) res = f(res, dat(left))
       if ((right & 1) == 0) res = f(res, dat(right))
       left = (left + 1) >> 1 // 右の子供なら右の親に移動
       right = (right - 1) >> 1 // 左の子供なら左の親に移動
     }

     res
   }
 }
}

class Main(out: java.io.PrintWriter, sc: Main.InputReader) {
  import sc._
  import Main._
  import java.util.Arrays.sort

  import scala.collection.mutable
  import math.{abs, max, min}
  import mutable.ArrayBuffer
  import Workspace._

  // toIntとか+7とかするならvalにしろ
  final private[this] val MOD = 1000000007

  def solve(): Unit = {
    import java.util.Comparator
    val n, m = ni()
    val E = Array.ofDim[Entry](m)
    REP(m) { i =>
      E(i) = Entry(ni() - 1, ni() - 1, ni())
    }

    // lで昇順
    sort(E, new Comparator[Entry] {
      override def compare(o1: Entry, o2: Entry): Int = Integer.compare(o1.l,o2.l)
    })

    val inf = 1e18.toLong + 10
    val v = Array.fill[Long](n)(inf)
    val seg = new SegmentTree(n, inf)(min)
    seg.update(0, 0)
    v(0) = 0
    REP(m) { i =>
      val from = seg.query(E(i).l, n)
      val next = from + E(i).c
      if (next < v(E(i).r)) {
        v(E(i).r) = next
        seg.update(E(i).r, next)
      }
    }
    val ans = seg.query(n - 1, n)
    if (ans == inf) out.println(-1)
    else out.println(ans)
  }
}

Measured memory_bytes: 53792