Naftogaz demands $18 billion including interest. This is about difference in price paid by Ukrainian company and price on the European market. The count of claim begins the moment Naftogaz asked Gazprom to revise the price for the first time, it was the highest in Europe at that time and consequently became almost two times higher that average price in Europe.
The request to renegotiate the price by one of the parties is provided for in the contract. However, it is now made clear in the contract how another side should react to this. Gazprom declined price revisions.
Naftogaz is not buying fuel from the Russian monopolist since 2016.