fn main() {
	let mut s = String::new();
	std::io::stdin().read_line(&mut s).ok();
	let n: Vec<usize> = s.split_whitespace().flat_map(str::parse).collect();
	println!(
		"{}",
		match n[0] {
			x if n[1] < 1 || x < n[1] => 0,
			x if x % 2 == 1 && n[1] == x / 2 + 1 => x - 1,
			x => x - 2,
		}
	);
}