import java.util.*;
public class main{
    public 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>40){
        System.out.println("Invalid input");
        return;
    }
    int res = fib(n);
    System.out.println(res);
}
}