#include<bits/stdc++.h>
using namespace std;
int main(){
	int a;
	float b;
	cin>>a;
	if(a<=10){
	b=2.5;
	}else
	{
	b=1.5+(a-10)*2.5;
	}
	printf("%.2f",b);
	return 0;
}

/**************************************************************
	Problem: 1043
	User: fuhoubin
	Language: C++
	Result: Accepted
	Time:19 ms
	Memory:2072 kb
****************************************************************/