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