120%AC

#include<cstdio>
double f(double x){
    if(x<5)return -x+2.5;
    else if(x<10)return 2-1.5*(x-3)*(x-3);
    else if(x<20)return x/2-1.5;
    else return false;
}
int main(){
    double x;
    scanf("%lf",&x);
    printf("%.3lf",f(x));
}