#include <iostream> #include <cstdio> 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("%.3f",y); return 0; }