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