#include <iostream> using namespace std; int main(){ double n; cin>>n; if(0<=n&&n<5) printf("%.3lf",-n2.5); eise if(5<=n&&n<10) printf("%.3lf",2-1.5*(n-3)*2); eise if(10<=n&&n<20) printf("%.3lf",n/2-1.5; return 0; }