#include <iostream>
#include <iomanip>
using namespace std;
 
int main() {
    double fee;
    cin >> fee;
     
    double weight;
    if (fee <= 15.0) {
        weight = fee / 1.5;
    } else {
        weight = 10 + (fee - 15.0) / 2.5;
    }
     
    cout << fixed << setprecision(2) << weight << endl;
    return 0;
}    
/**************************************************************
	Problem: 1052
	User: baizhoudeyueguang
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/