#include<bits/stdc++.h>
using namespace std;
int main(){
	int a;
	cin>>a;
	if(a<=10) printf("%.2f",2.5);
	else printf("%.2f",2.5+((a-10)*1.5));
	
}
/**************************************************************
	Problem: 1043
	User: chenjunlin
	Language: C++
	Result: Accepted
	Time:15 ms
	Memory:2072 kb
****************************************************************/