pre_text: The following tables summarize the activity related to deferred revenue and financed unearned services revenue (in millions): During the years ended April 26, 2019 and April 27, 2018, we recognized $1,722 million and $1,806 million, respectively, that was included in the deferred revenue and financed unearned services revenue balance at the beginning of the respective periods. As of April 26, 2019, the aggregate amount of the transaction price allocated to the remaining performance obligations related to customer contracts that are unsatisfied or partially unsatisfied was $3,668 million, which is equivalent to our deferred revenue and unearned services revenue balance. Because customer orders are typically placed on an as-needed basis, and cancellable without penalty prior to shipment, orders in backlog may not be a meaningful indicator of future revenue and have not been included in this amount. We expect to recognize as revenue approximately 50% of our deferred revenue and financed unearned services revenue balance in the next 12 months, approximately 25% in the next 13 to 24 months, and the remainder thereafter.
table: Year Ended
table: April 26, 2019 April 27, 2018
table: Balance at beginning of period $  3,363 $ 3,213
table: Additions 2,763 2,566
table: Revenue recognized during the period (2,458 ) (2,416 )
table: Balance at end of period $  3,668 $ 3,363
question: Analyse this data from a financial earnings document. What was the change in the balance at beginning of period between 2018 and 2019?