import java.util.*;

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