fun main(){
    val n = readLine()!!.toInt()
    var pos = mutableListOf<Pair<Int,Int>>()
    repeat(n){
        val (x, y) = readLine()!!.split(" ").map{it.toInt()}
        pos.add(Pair(x,y))
    }
    val (x1, y1) = pos[0]
    println(pos.zipWithNext{p2, p3 -> Math.abs((x1-p3.first)*(p2.second-p3.second)-(p2.first-p3.first)*(y1-p3.second))}.sum())
}