#include<iostream> 
#include<cstdio> 
#include<cmath> 
#include<algorithm>
using namespace std;
int main(){
	double n,y;
	cin >> n;
	if(0 <= n && n < 5){
		y = -n+2.5;
		printf("%.3lf",y);
	}else if(5 <= n && n < 10){
		y = 2-(1.5*(n-3)*(n-3));
		printf("%.3lf",y);
	}else if(10 <= n && n < 20){
		y = n/2-1.5;
		printf("%.3lf",y);
	}
	return 0;
}

#include<iostream>
#include<cstdio>
using namespace std;
double a,b;
int main(){
	cin>>a;
	if(a>=0&&a<5) b=-a+2.5;
	else if(a>=5&&a<10) b=2-1.5*(a-3)*(a-3);
	else if(a>=10&&a<20) b=a/2.0-1.5;
	printf("%.3lf",b);
	return 0;
}

https://www.luogu.com.cn/record/68077585

#include <bits/stdc++.h>
using namespace std;
int main()
{
    double n;
    cin>>n;
    if(n>=0&&n<5)printf("%.3f",0-n+2.5);
    if(n>=5&&n<10)printf("%.3f",2-1.5*(n-3)*(n-3));
    if(n>=10&&n<20)printf("%.3f",n/2-1.5);
    return 0;
}