use std::io::Read;

fn main() {
	let mut s = String::new();
	std::io::stdin().read_to_string(&mut s).ok();
	let n: Vec<i64> = s.split_whitespace().flat_map(str::parse).collect();
	let mut a = 0;
	for v in n[3..].windows(4).step_by(2) {
		a += (v[0] - n[1]) * (v[3] - n[2]) - (v[1] - n[2]) * (v[2] - n[1]);
	}
	println!("{a}")
}