fn main() {
	let mut n = String::new();
	std::io::stdin().read_line(&mut n).ok();
	let mut n: u64 = n.trim().parse().unwrap();
	let (mut a, mut b, mut c) = (1u64, 0u64, 0u64);
	for _ in 0..n {
		c = a + b;
		b = a;
		a = c;
	}
	println!("{}", c);
}