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 sc = new Scanner(System.in);
        if(!sc.nextInt()){
            System.out.println("Invalid input");
        }
        int num = sc.nextInt();
        System.out.println(fun(num));
    }
}