斐波那契通项公式

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        if(n > 20) 
            System.out.println(0.61803399);
        else
            System.out.printf("%.8f", fib(n) / fib(n + 1));
    }

    public static double fib(int n) {
        double x = Math.pow(5, 0.5);
        return (Math.pow((1 + x) / 2, n) - Math.pow((1 - x) / 2, n)) / x;
    }
}