import scala.io.StdIn._

import Math._
object Main extends App {
  val eps0 = 1e-12

  val q = readLine.toInt

  val xs = new Array[Int](q)
  val ys = new Array[Int](q)
  for (i <- 0 until q) {
    val Seq(x, y) = readLine.split(' ').map(_.toInt).toList
    xs(i) = x
    ys(i) = y
  }

  val subarea = for (i <- 2 until q) yield {
    val xs1 = xs(i-1) - xs(0)
    val ys1 = ys(i-1) - ys(0)
    val xs2 = xs(i) - xs(0)
    val ys2 = ys(i) - ys(0)

    xs1 * ys2 - ys1 * xs2
  }

  println("%.1f".format(subarea.sum.toDouble / 2))
}
