lz

#include <iostream>
#include <iomanip>

int main() {
    double x;
    std::cin >> x;

    double y;
    if (0 <= x && x < 5) {
        y = -x + 2.5;
    } else if (5 <= x && x < 10) {
        y = 2 - 1.5 * (x - 3) * (x - 3);
    } else if (10 <= x && x < 20) {
        y = x / 2 - 1.5;
    }

    std::cout << std::fixed << std::setprecision(3) << y << std::endl;

    return 0;
}