#include <iostream>

using namespace std;

class Cena
{
    friend double operator+(Cena&, double);
    friend double operator-(Cena&, double);
    double PLN;
public:
    Cena(double);
};

double operator+(Cena& cena, double kwota)
{
    kwota *= 1.1;
    cena.PLN += kwota;
    return cena.PLN;
}

double operator-(Cena& cena, double kwota)
{
    kwota * 0.95;
    cena.PLN -= kwota;
    if(cena.PLN < 0)
        cena.PLN = 0;
    return cena.PLN;
}

Cena::Cena(double ile) : PLN(ile) {}

int main()
{
    Cena cena(20);
    cout << cena + 15 << endl;
    cout << cena - 12 << endl;
    return 0;
}