func main()
	var n: int :: cui@inputInt()
	var x: []int :: #[n]int
	var y: []int :: #[n]int
	for i(0, n - 1)
		do x[i] :: cui@inputInt()
		do y[i] :: cui@inputInt()
	end for
	
	var ans: int :: 0
	for i(1, n - 2)
		do ans :+ f(x[0], y[0], x[i], y[i], x[i + 1], y[i + 1])
	end for
	do cui@print("\{ans}\n")
	
	func f(x1: int, y1: int, x2: int, y2: int, x3: int, y3: int): int
		var res: int :: 0
		ret((x3 - x1) * (y2 - y1) - (y3 - y1) * (x2 - x1)).abs()
	end func
end func