import java.util.Scanner;

public class No44 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		long n1 = 1;
		long n2 = 0;
		long n = 2;
		for(int i = 3;i <= N;i++) {
			n2 = n1;
			n1 = n;
			n = n1 + n2;
		}
		System.out.println(n);
	}
}