package exercise_java;

import java.util.Scanner;


public class CircleArea {
	public static void main(String[] args){
		Scanner in = new Scanner(System.in);
		
		System.out.println("please input the radius: ");
		
		double radius = in.nextDouble();
		double semiarea =radius*radius*Math.PI/2;
		
		System.out.println("the area of semi-circle is"+semiarea);
	}

}