import java.util.Scanner;


public class das {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        long mas[] = new long[46];
        mas[0] = 2;
        mas[1] = 2;

         for(int i=2;i<46;i++){
              mas[i] = mas[i-1]+mas[i-2];
          }

        System.out.print(mas[n-1]);

    }

}