import math._

object Main {
  def main(args: Array[String]) {
    val sc = new java.util.Scanner(System.in)
    val n = sc.nextInt
    val xs = (1 to n) map { _ =>
      val a, b = sc.nextInt
      a + 4 * b
    }

    val max = xs.max
    var ans = 0L
    for (x <- xs) {
      if ((max - x) % 2 == 1) {
        println(-1)
        return
      }
      ans += (max - x) / 2
    }
    println(ans)
  }
}