func main()
	var n: int :: cui@inputInt()
	var x0: int :: cui@inputInt()
	var y0: int :: cui@inputInt()
	var x1: int :: cui@inputInt()
	var y1: int :: cui@inputInt()
	var ans: int :: 0
	for i(1, n - 2)
		var x2: int :: cui@inputInt()
		var y2: int :: cui@inputInt()
		do ans :+ (x1 - x0) * (y2 - y0) - (y1 - y0) * (x2 - x0)
		do x1 :: x2
		do y1 :: y2
	end for
	do cui@print("\{ans}\n")
end func