#include <iostream>
#include <iomanip>
using namespace std;
int main(){
	double d;
	cin>>d;
	cout<<fixed<<setprecision(2)<<(d*d)-0.25*3.14*d*d;
}

六上基础题目

#include<iostream>//cin cout
#include<iomanip>//setprecision setw
#include<stdio.h>//scanf printf
#include<math.h>//ceil floor fabs sqrt round pow
#include<string>//string
#include<string.h>//memset
#include<sstream>//stringsream
#include<algorithm>//min max
using namespace std;
double a,d;
int main(){
	cin>>d;
	a=d*d/4*0.86;
	printf("%.2lf",a);
	return 0;
}

#include <iostream>
#include <iomanip>
using namespace std;
int main(){
	double d;
	cin>>d;
	cout<<fixed<<setprecision(2)<<(d*d)-0.25*3.14*d*d;
}