function correctPrices() {
    if (document.forms[0].cena_od.selectedIndex != 0 && document.forms[0].cena_do.selectedIndex != 0) {
        if (document.forms[0].cena_do.selectedIndex < document.forms[0].cena_od.selectedIndex) {
            document.forms[0].cena_do.selectedIndex = document.forms[0].cena_od.selectedIndex;
        }
        if (document.forms[0].cena_od.selectedIndex > document.forms[0].cena_do.selectedIndex) {
            document.forms[0].cena_do.selectedIndex = document.forms[0].cena_od.selectedIndex;
        }
    }
}