fun main(args:Array<String>) {
  val n = readLine()!!.toInt()
  val sizes = (1..n).map {
    val (a, b) = readLine()!!.split(" ").map(String::toInt)
    a+4*b
  }

  val min = sizes.min()!!
  val deltas = sizes.map { it - min }

  when {
    deltas.any { it%2 != 0 } -> -1
    else -> {
      deltas.sum()/2
    }
  }.run(::println)
}