14.5.4. Change in liabilities from contracts with customers
| 31/12/2021 | 31/12/2020 | |
| At the beginning of the period | 453 | 246 |
| Revenues recognised in a given reporting period, included in the balance of liabilities from contracts with customers at the beginning of the period | (166) | (141) |
| Acquisition of subsidiaries | 5 | 112 |
| Revenue adjustments | 61 | 74 |
| Advances received, prepayments | 371 | 162 |
| Foreign exchange differences | 4 | – |
| 728 | 453 |
The Group fulfils its liabilities from contracts with customers with respect to advance payments received, prepayments up to one year. Revenues adjustments are related to deferred part of revenue related to the loyalty program VITAY, according to which the customer is entitled to future benefits (so-called VITAY points). Points are valid for 3 years from the date they were obtained. During this period, the Group expects to satisfy a performance obligation by exchanging collected points for transferred goods / services to customers and recognise revenues.