import java.util.Scanner

object Problem185 {
  def main(args: Array[String]) {
    val sc = new Scanner(System.in)
    val N = sc.nextInt
    val XY = Seq.fill(N)((sc.nextInt, sc.nextInt))

    val answer = XY.head._2 - XY.head._1

    val result = XY.forall(xy => xy._2 - xy._1 == answer) match {
      case _ if answer <= 0 => -1
      case true => answer
      case false => -1
    }

    println(result)
  }
}