fn main() {
	let mut s = String::new();
	std::io::stdin().read_line(&mut s).ok();
	let n: u32 = s.trim().parse().unwrap();
	let a = if n == 2 {
		2
	} else {
		(n - 3) / 2 * 10 + 4 + n % 2
	};
	println!("{}", a);
}