import scala.collection.mutable.ListBuffer

object Main {
  val sc = new java.util.Scanner(System.in)
  def readLine(): String = sc.nextLine
  def readInts(): Array[Int] = sc.nextLine.split(' ').map(_.toInt)
  def readInt(): Int = sc.nextLine.toInt

  def calc(xs: ListBuffer[(Int, Int)]) {
    val n = xs.head._2 - xs.head._1
    if (n <= 0) {
      println(-1)
      return
    }

    for ((x, y) <- xs.tail) {
      if (n != y - x) {
        println(-1)
        return
      }
    }
    println(n)
  }

  def main(args: Array[String]) {
    val n = readInt
    val xs = new ListBuffer[(Int, Int)]
    for (i <- 0 to n-1) {
      val Array(x, y) = readInts
      xs.append((x, y))
    }
    calc(xs)
  }
}