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 val oj = System.getenv("ATCODER_DEBUG") == null
  def DEBUG(f: => Unit): Unit = {
    if (!oj){ f }
  }
  def debug(as: Array[Boolean]): Unit = if (!oj){ debug(as.map(x => if(x) "1" else "0").mkString) }
  def debug(as: Array[Int]): Unit = if (!oj){ debug(as.mkString(" ")) }
  def debug(as: Array[Long]): Unit =if (!oj){ debug(as.mkString(" ")) }
  def debugDim(m: Array[Array[Long]]): Unit = if (!oj){
    REP(m.length) { i =>
      debug(m(i))
    }
  }
  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()
    }
  }
  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 = 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) { 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]): Array[Long] = {
    val cum = Array.ofDim[Long](as.length + 1)
    REP(as.length) { i =>
      cum(i + 1) = cum(i) + as(i)
    }
    cum
  }
}

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

  // toIntとか+7とかするならvalにしろ
  @inline private def MOD = 1000000007

  def solve(): Unit = {
    import java.util
    val N = ni()
    val A = Array.fill[util.ArrayDeque[Int]](N)(new util.ArrayDeque[Int])
    REP(N) { i =>
      REP(N - 1) { _ =>
        A(i).add(ni() - 1)
      }
    }
    DEBUG {
      import scala.collection.JavaConverters._
      REP(N) { i =>
        debug(A(i).asScala.mkString(" "))
      }
    }

    var t = 0
    var empties = 0
    val deleted = Array.ofDim[Boolean](N)
    val q = Array.fill[Stack](2)(new Stack(N))
    REP(N) { i =>
      q(0) += i
    }
    while(empties < N) {
      val x= t&1
      import java.util
      util.Arrays.fill(deleted, false)
      while(!q(x).isEmpty) {
        val i = q(x).pop()
        if (!A(i).isEmpty) {
          val j = A(i).peek
          if (A(j).peek == i) {
            if (!deleted(i)) {
              deleted(i) = true
              q(x^1) += i
            }
            if (!deleted(j)) {
              deleted(j) = true
              q(x^1) += j
            }
          }
        }
      }

      var ok = false
      REP(q(x^1).length) { ii =>
        val i = q(x^1)(ii)
        deleted(i) = false
        ok = true
        A(i).poll()
        if (A(i).isEmpty) empties += 1
      }
      if (!ok){
        out.println(-1)
        return
      }
      t += 1
      debug(s"empties:$empties")
    }

    out.println(t)
  }

  class Stack(n: Int) {
  private val as = Array.ofDim[Int](n)
  var p = 0

  def +=(a: Int): Unit = {
    as(p) = a
    p += 1
  }

  def pop(): Int = {
    p -= 1
    as(p)
  }

  def peek: Int = as(p - 1)

  def apply(i: Int): Int = as(i)

  def length: Int = p
  def isEmpty: Boolean = length == 0
  def nonEmpty: Boolean = !isEmpty

  def mkString(delimiter: String = ""): String = as.take(p).mkString(delimiter)
}
}

Measured memory_bytes: 82392