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