#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; double x; int main(){ cin>>x; if(x<5){ cout<<x*-1+2.5; }else if(x<10){ cout<<2-1.5*(x-3)*2; }else{ cout<< x/2-1.5; } return 0; }