public class Main {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n;
        while(sc.hasNext()){
        n=sc.nextInt();    
        System.out.println(sum(n));
        }
        sc.close();
    }
    public static int sum(int i){
        if(i<=4){
            return i;
        }else{
            return sum(i-1)+sum(i-3);
        }
    }
}