#include<bits/stdc++.h>
using namespace std;
int main() {
	long double x;
	cin >> x ;
	cout<< fixed << setprecision(3) ;
	if(x < 5) cout<< -x + 2.5;
	else if(x < 10) cout<< 2 - 1.5 *(x-3) * (x-3);
	else cout<< x / 2 - 1.5;
}