C :

#include <stdio.h>
#include <math.h>
int main()
{
    int r;
	double s, PI;
	while (scanf("%d", &r) != EOF){
	PI = atan(1.0) * 4;
    s = PI * r * r;
    printf("%.7lf\n", s);
	}
    return 0;
}

C++ :

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<math.h>

int main()
{

int m;
	double s=0;
	double PI;
	//tan(PI/4)==1;
	PI=atan(1)*4;
	while(scanf("%d",&m)!=EOF)
	{
 
		s=PI*m*m;

        printf("%.7f\n",s);

	}


return 0;

}

Java :

import java.util.Scanner;


public class Main {

	public static void main(String[] args) {
		final double PI=Math.PI;
		Scanner s = new Scanner(System.in);
		while(s.hasNext()){
		double r = s.nextDouble();
		System.out.println(String.format("%.7f", PI*r*r));
			
		}

	}

}