package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
)

const MaxInt = math.MaxInt64

var scanner *bufio.Scanner

func abs(a int) int {
	if a < 0 {
		return -a
	}
	return a
}

func main() {
	n := Int()
	x, y := Int(), Int()
	ptsX := make([]int, n-1)
	ptsY := make([]int, n-1)
	for i := 0; i < n-1; i++ {
		ptsX[i] = Int() - x
		ptsY[i] = Int() - y
	}
	ans := 0
	for i := 0; i < n-2; i++ {
		area2 := abs(ptsX[i]*ptsY[i+1] - ptsX[i+1]*ptsY[i])
		ans += area2
	}
	fmt.Println(ans)
}

func init() {
	scanner = bufio.NewScanner(os.Stdin)
	scanner.Buffer([]byte{}, MaxInt)
	scanner.Split(bufio.ScanWords)
}

func Int() int {
	scanner.Scan()
	ret, _ := strconv.ParseInt(scanner.Text(), 10, 0)
	return int(ret)
}

func Ints(n int) []int {
	ret := make([]int, n)
	for i := 0; i < n; i++ {
		ret[i] = Int()
	}
	return ret
}

func Max(a, b int) int {
	if a < b {
		return b
	}
	return a
}

func Min(a, b int) int {
	if a < b {
		return a
	}
	return b
}