import java.util.*;
public class Main{
    static int fib(int n){
        if(n==1) return 0;
        if(n==2) return 1;
        return fib(n-1)+fib(n-2);
    }
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        if(n<=0||!n.matches("\\d+")){
            System.out.print("Invalid input");
        }
        System.out.print(fib(n));
    }
}