题解:
#define _USE_MATH_DEFINES
#include<bits/stdc++.h>
#include<cmath>
using namespace std;
int main(){
int n;
double a,b,c,ans = 0.0;
cin>>n;
a = (n/4.0) * (n/4.0);
b = (n/(2 * M_PI)) * (n/(2*M_PI)) * M_PI;
c = ((n/3.0) * (n/3.0) * sqrt(3)) / 4.0;
ans = max({a,b,c});
printf("%.3f",ans);
}