Calculator: no error message when entering alphabetic characters into index fields

Steps to reproduce:

1. Open http://nibulon.com/data/zakupivlya-silgospprodukcii/kalkulyator-znizhok.html

2. Enter alphabetic characters into index fields.

3. Press "Розрахувати" button.

Expected result: error message is displayed "Only digits are allowed".

Actual result: zero calculation results.