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