import java.math.BigInteger;
import java.util.Scanner;
public class Main{
    public static int MAXN = 10000 + 10;
    public static BigInteger []c = new BigInteger[MAXN];
    public static void init(){
        c[3] = new BigInteger("4");
        c[4] = new BigInteger("7");
        for(int i = 5; i < MAXN; ++i)
            c[i] = c[i - 1].add(c[i - 2]);
    }
    public static void main(String []args){
        init();
        Scanner sc = new Scanner(System.in);
        while(sc.hasNextInt()){
            int n = sc.nextInt();
            System.out.println(c[n]);
        }
    }
}