/* scala -

to calculate the area of a polygon when its coordinates are given we ll use shoelace formula = 0.5 * ( x1y2 − x2y1 + x2y3 − x3y2 +. . . + xny0 − x0yn)

(x1,y1), (x2,y2) => x1y2 − x2y1 (x2,y2), (x3,y3) => x2y3 − x3y2 . . . (xn,yn), (x0,y0)=> xny0 − x0yn //firstPoint and last point

twoPointPairs is a list of sequence of pairs ie List[Seq[(Int, Int)]] from 1st point until last point. For first and last point part of shoelace formula will have to be calculated separately. */

object ComputeTheAreaOfAPolygon {

def partOfShoelaceFormula(p1: (Int, Int), p2: (Int, Int)): Double = {

val (x1, y1) = p1
val (x2, y2) = p2

(x1 * y2 - x2 * y1 )

}

def main(args: Array[String]): Unit = { val stdin = scala.io.StdIn

val n = stdin.readLine.trim.toInt

val points: Seq[(Int, Int)] = for (nItr <- 1 to n) yield { //points = Seq(p1, p2, ... ,pn) = Seq((x1, y1), (x2, y2), ... ,(xn, yn))
  val readline = stdin.readLine.split(" ").map(x => x.toInt) //split returns an array of string, map converts array of string to array of int
  val xCoordinate = readline(0) //accessing 0th element of array x
  val yCoordinate = readline(1)
  (xCoordinate, yCoordinate)
}

val twoPointPairs: List[Seq[(Int, Int)]] = points.sliding(2).toList // p2p3pairs: List[Seq[(Int, Int)]] every Seq here has 2 elements ie  List( Seq( (x1, y1), (x2, y2) ), Seq( (x2, y2), (x3, y3) ) ,... , Seq( (xn-1, yn-1), (xn, yn) ) )

val areaOfPolygon = 0.5 * (twoPointPairs.map {
  case Seq(p1, p2) => partOfShoelaceFormula(p1, p2)
}.sum + partOfShoelaceFormula(points.last, points.head)) //here (lastPoint, firstPoint) should be the order
println(areaOfPolygon)

} }

println(areaOfPolygon)

} }