import java.util.Scanner;

public class Fibonacci {
    public static void main(String[] args) {
        Scanner myScanner=new Scanner(System.in);
        int a=myScanner.nextInt();
        double b=fibonacci(a);
        System.out.printf("%.2f",b);
    }
    public static int fibonacci(int n){
        if(n<=1){
            return n;
        }else {
            return fibonacci(n-1)+fibonacci(n-2);
        }
    }
}