#include<bits/stdc++.h>
using namespace std;
 
int main() {
     double fare;
    cin >> fare;
    double weight;
    if (fare <= 1.5 * 10) {
        weight = fare / 1.5;
    } else {
        weight = 10 + (fare - 1.5 * 10) / 2.5;
    }
    cout << fixed << setprecision(2) << weight << endl;
}
/**************************************************************
	Problem: 1052
	User: yangjunwen
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/