#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(){ int l; cin>>l; if(l<=2){ cout<<"6"; } else if(l<=10){ cout<<(l-2)*1.8; } else { cout<<6+(10-2)*1.8+(l-10)*(1.8+(1.8*0.5)); } return 0; }