# include<iostream> using namespace std; int main(){ double n,y; cin>>n; if(n>=0&&n<5) y=-n+2.5; if(n>=5&&n<10) y=2-1.5*(n-3)*2; if(n>=10&&n<20) y=n/2-1.5; printf("%.3lf",y);//格式控制字符串，输出表列 return 0; }