import java.util.*;
public class main{
    static int fun(int num)
    {
        if(num==1)
        {
            return 0;
        }
        if(num==2)
            return 1;
        return fun(num-1)+fun(num-2);
    }
    public static void main(String[] args)
    {
        Scanner scan=new Scanner(System.in);
        int num=scan.nextInt();
        if(n<0)
        {
            System.out.println("Invalid input");
        }
        System.out.println(fun(num));
    }
}