import java.util.Scanner;
class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int num = sc.nextInt();
		long [] t = new long [] {2,1};
		for(int i = 3 ; i <= num ; i++) t[i%2] = t[0] + t[1];
		System.out.println(t[num % 2]);
	}
}