LQB:圆的面积

#include<iostream>
#include<cstdio>
#include<cmath>
using namespace std;
#define R 10000
int main(){
	int r;
	cin>>r;
	double PI = atan(1.0)*4;  //求PI的公式
	double result=PI*pow(r,2);  //cmath
	printf("%.7f",result);   //cstdio
	return 0;
}