package main

import . "fmt"
import . "math/cmplx"
import m "math"

func main() {
	var n int
	Scan(&n)
	a:=make([]complex128, n)
	for i := range a{
		var x,y float64
		Scan(&x,&y)
		a[i] = Rect(x,0)+Rect(y,0)*1i
	}
	a=append(a,a[0])
	var ans float64
	for i,v := range a[1:] {
		ans += real(a[i])*imag(v)-imag(a[i])*real(v)
	}
	Println(m.Abs(ans))
}