object Main {
  def main(args: Array[String]): Unit = {
    val s = new Main()
    s.solve()
    s.out.flush()
  }
}

class Main {
  import java.io._
  import java.util.StringTokenizer
  import java.util.Arrays

  import scala.collection.mutable
  import scala.util.Sorting
  import math.{abs, max, min}
  import mutable.{ArrayBuffer, ListBuffer}
  import scala.reflect.ClassTag

  val MOD = 1000000007
  val out = new PrintWriter(System.out)

  def solve(): Unit = {
    val N, M = ni()
    val from, to, S = Array.ofDim[Int](M)
    REP(M) { i =>
      val u, v = ni() - 1
      val s = ni()
      from(i) = u
      to(i) = v
      S(i) = s
    }

    val g = packWUGraph(N, from, to, S)

    val (_, _, q) = traceBfs(g)
    val rg = Array.ofDim[Int](2, N) // [from, to]
    Arrays.fill(rg(0), 1)
    Arrays.fill(rg(1), 1e9.toInt) // 無限みたいなもの

    // 適当なツリーを２周する
    REP(2) { _ =>
      REP_r(N) { i =>
        val v = q(i)
        var (l, r) = (rg(0)(v), rg(1)(v))
        REP(g(v).length) { j =>
          val u = g(v)(j)
          l = max(l, u.s - rg(1)(u.to))
          r = min(r, u.s - rg(0)(u.to))
        }
        debug(s"visit($v) [$l,$r]")

        // Longにするだけでいいのかもしれないけどありえない状態を表現しておく
        if (l > r) {
          rg(0)(v) = 0
          rg(1)(v) = -1
        } else {
          rg(0)(v) = l
          rg(1)(v) = r
        }
      }
    }

    debug(rg(0))
    debug(rg(1))

    val ans = map(N) { i =>
      rg(1)(i) - rg(0)(i) + 1
    }.min

    if (ans <= 0) out.println(0)
    else out.println(ans)
  }

  case class Edge(to: Int, s: Int)
  type WUGraph = Array[Array[Edge]]
  /**
    * uwiのぱくり
    */
  def packWUGraph(n: Int, from: Array[Int], to: Array[Int], s: Array[Int]): WUGraph = {
    val g = new Array[Array[Edge]](n)
    val p = new Array[Int](n)
    val m = from.length
    REP(m)(i => p(from(i)) += 1)
    REP(m)(i => p(to(i)) += 1)
    REP(n)(i => g(i) = new Array(p(i)))
    REP(m) { i =>
      p(from(i)) -= 1
      g(from(i))(p(from(i))) = Edge(to(i), s(i))
      p(to(i)) -= 1
      g(to(i))(p(to(i))) = Edge(from(i), s(i))
    }
    g
  }


  /**
    * @return (depth, parent, queue)
    */
  def traceBfs(g: WUGraph, rt: Int = 0): (Array[Int], Array[Int], Array[Int]) = {
    val n = g.length
    val INF = 1e9.toInt + 10
    val q, p = Array.ofDim[Int](n)
    q(0) = rt
    p(rt) = -1
    val d = Array.fill[Int](n)(INF)
    d(rt) = 0
    var cur = 0
    var last = 1
    while (cur < last) {
      val v = q(cur)
      REP(g(v).length) { i =>
        val u = g(v)(i)
        if (d(u.to) == INF) {
          d(u.to) = d(v) + 1
          p(u.to) = v
          q(last) = u.to
          last += 1
        }
      }
      cur += 1
    }
    (d, p, q)
  }

  private val oj = System.getenv("ATCODER_DEBUG") == null

  def DEBUG(f: => Unit): Unit = {
    if (!oj){ f }
  }

  def debug(as: Array[Boolean]): Unit = DEBUG {
    debug(as.map(x => if(x) "1" else "0").mkString)
  }

  def debug(as: Array[Int]): Unit = DEBUG {
    debug(as.mkString(" "))
  }

  def debug(as: Array[Long]): Unit = DEBUG {
    debug(as.mkString(" "))
  }

  def debug(s: => String): Unit = DEBUG {
    System.err.println(s)
  }

  def debugL(num: => Long): Unit = DEBUG {
    System.err.println(num)
  }

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

    def next(): String = {
      while (tokenizer == null || !tokenizer.hasMoreTokens)
        tokenizer = new StringTokenizer(reader.readLine)
      tokenizer.nextToken
    }

    def nextInt(): Int = next().toInt
    def nextLong(): Long = next().toLong
    def nextChar(): Char = next().charAt(0)
  }
  val sc = new InputReader(System.in)
  def ni(): Int = sc.nextInt()
  def nl(): Long = sc.nextLong()
  def nc(): Char = sc.nextChar()
  def ns(): String = sc.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) { i =>
      f(i)
    }
  }

  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]) = {
    val cum = Array.ofDim[Long](as.length + 1)
    REP(as.length) { i =>
      cum(i + 1) = cum(i) + as(i)
    }
    cum
  }
}
