#include <iostream>
#include <cstring>
#include <algorithm>
#include <string>
#include <iomanip>
#include <cmath>
using namespace std;
bool A[1145][1145];
int main(){int cnt;
	double n,m;
	cin >>n;
	if(n>=0&&n<5)cout<<fixed<<setprecision(3)<<0.1*n+2.5;
	else if(5<=n&&n<10)cout<<fixed<<setprecision(3)<<-1.5*(n-3)*(n-3);
	else if(n>=10&&n<20)cout<<fixed<<setprecision(3)<<n/2-1.5;
	
}