import java.util.Scanner;

public class N44
{
	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		long[] ans=new long[a];
		ans[0]=1;
		ans[1]=2;
		for(int i=2;i<a;i++)
		{
			ans[i]=ans[i-2]+ans[i-1];
		}
		System.out.println(ans[a-1]);
	}

}