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 b = 0;
	for a in &n[2..] {
		b += n[1] - a;
	}
	println!("{}", n[1] - b)
}