一本通1051

//qf
#include <bits/stdc++.h>
#define f(i,j,n) for(register int i=j;i<=n;i++)
using namespace std;
typedef long long ll;
inline ll read() {
	ll x=0;
	int f=1;
	char ch=getchar();
	while(!isdigit(ch)) {
		if(ch=='-') f=-1;
		ch=getchar();
	}
	while(isdigit(ch)) x=(x<<1)+(x<<3)+(ch-48),ch=getchar();
	return x*f;
}
signed main() {
	double x,y;
	cin>>x;
	if(0<=x and x<5) y=-x+2.5;
	else if(5<=x and x<10) y=2-1.5*(x-3)*(x-3);
	else y=x/2-1.5;
	cout<<fixed<<setprecision(3)<<y<<endl;
	return 0;
}