package EAREA
 
import kotlin.math.abs
 
fun main(args: Array<String>) {
    if(args.isNotEmpty()) {
        System.setIn(java.io.FileInputStream(args[0]))
    }
    if(args.size >= 2) {
        System.setOut(java.io.PrintStream(args[1]))
    }
 
    val N = readLine()!!.toInt()
    require(N in 1..100000)
 
    data class Point(val x: Long, val y: Long)
    val points = List(N) {
        val (x, y) = readLine()!!.split(" ").map{ it.toLong() }
        require(abs(x) <= 10000000)
        require(abs(y) <= 10000000)
        Point(x, y)
    }
 
    fun expectedAreaTimes8(a: Point, b: Point, c: Point, d: Point, e: Point, f: Point): Long {
        return c.x * a.y * -1 + c.x * b.y * -1 + c.y * a.x * 1 + c.y * b.x * 1 + d.x * a.y * -1 + d.x * b.y * -1 + d.y * a.x * 1 + d.y * b.x * 1 + e.x * a.y * 1 + e.x * b.y * 1 + e.x * c.y * -1 + e.x * d.y * -1 + e.y * a.x * -1 + e.y * b.x * -1 + e.y * c.x * 1 + e.y * d.x * 1 + f.x * a.y * 1 + f.x * b.y * 1 + f.x * c.y * -1 + f.x * d.y * -1 + f.y * a.x * -1 + f.y * b.x * -1 + f.y * c.x * 1 + f.y * d.x * 1
    }
 
    val ansTimesEight = (1 until N-1).map { i -> expectedAreaTimes8(points[0], points[1], points[i], points[i+1], points[i+1], points[(i+2)%N]) }.sum()
    print(ansTimesEight / 8)
    print(".")
    println(ansTimesEight % 8 * 125)
 
    require(readLine() == null)
}

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