#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=0,y; int a,b; cin>>x; if(0<=x && x<5){ y=-x+2.5; } else if(5<=x && x<10){ y=2-1.5*(x-3)*2; } else if(10<=x && x<20){ y=x/2-1.5; } printf("%.3lf",y); return 0; }