import java.util.Scanner;

public class DPsugoroku {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int N = s.nextInt();
		s.close();
		long[] root = new long[N+1];
		root[1] = 1;
		root[2] = 2;
		for(int i =3;i <= N;i++){
			root[i] = root[i-1] + root[i-2];
		}
		System.out.println(root[N]);

	}

}