#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    int weight;
    cin >> weight;
    double fee;
    if (weight <= 10) {
        fee = 2.5;
    } else {
        fee = 2.5+(weight - 10)*1.5;
    }
    cout << fixed << setprecision(2) << fee << endl;
    return 0;
}    
/**************************************************************
	Problem: 1043
	User: fuyijun
	Language: C++
	Result: Accepted
	Time:13 ms
	Memory:2072 kb
****************************************************************/