#include<bits/stdc++.h>
using namespace std;
int main(){
	int a;
	
	cin>>a;
	if(a<=10)
		printf("%.2f",2.5);
		
	if(a>10)
		printf("%.2f",2.5+(a-10)*1.5);	
		
	return 0;
}
/**************************************************************
	Problem: 1043
	User: cyp
	Language: C++
	Result: Accepted
	Time:14 ms
	Memory:2072 kb
****************************************************************/