import java.util.Scanner

object Problem178 {

  def proc(AB: Seq[(Int, Int)]): Long = {
    val width = AB.map(x => x._1 + x._2 * 4L)
    if (!width.forall(x => x % 2 == width.head % 2)) {
      return -1
    }

    val maxWidth = width.max
    width.map(x => (maxWidth - x) / 2).sum
  }

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

    val result = proc(AB)
    println(result)
  }
}