题解 | #牛牛的圆#

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        //定义圆周率为3.14
        double PI = 3.14;
        double r = scanner.nextDouble();
        
        System.out.println(r * r * PI);
        
    }
}