$C++$

#include <bits/stdc++.h>
using namespace std;
const double pi=3.14;
int main() {
    double a;
    //freopen("vidu.inp","r",stdin);
    //freopen("vidu.out","w",stdout);
    cin>>a;
    double dientichtamgiac=sqrt(3)/4*a*a;
    double r=a/(2*sqrt(3));
    double dintichhinhtron=pi*r*r;
    double dientichphangachcheo=dientichtamgiac-dintichhinhtron;
    cout<<fixed<<setprecision(2)<<dientichphangachcheo;
    return 0;
}