var change_form_structure = function change_form_structure(element) {
    var input_nip = document.getElementById('register_form_nip');
    var input_company = document.getElementById('register_form_company');
    if(element.value == 'f') {
        input_company.disabled = input_nip.disabled = false;
    } else {
        input_company.disabled = input_nip.disabled = true;
    }
}