#include<iostream> // cin\cout\endl #include<cstdio> //scanf()\printf() #include<cstring> // strcpy()\strcat()\strcmp()\strlen()\memset() #include<cmath> //sqrt()\pow()\abs()\ceil()\floor()\max()\min() 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("%.3lf\n",y); return 0; }