#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
double i,j,k;
while(cin>>i){
k=(i*i)/4;
cout<<fixed<<setprecision(3)<<k<<endl;
}
return 0;
}
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
double i,j,k;
while(cin>>i){
k=(i*i)/4;
cout<<fixed<<setprecision(3)<<k<<endl;
}
return 0;
}