Detailed Instructions: This task involves creating answers to complex questions, from a given passage. Answering these questions, typically involve understanding multiple sentences. Make sure that your answer has the same type as the "answer type" mentioned in input. The provided "answer type" can be of any of the following types: "span", "date", "number". A "span" answer is a continuous phrase taken directly from the passage or question. You can directly copy-paste the text from the passage or the question for span type answers. If you find multiple spans, please add them all as a comma separated list. Please restrict each span to five words. A "number" type answer can include a digit specifying an actual value. For "date" type answers, use DD MM YYYY format e.g. 11 Jan 1992. If full date is not available in the passage you can write partial date such as 1992 or Jan 1992. If you find multiple spans, please add them all as a comma separated list. Please restrict each span to five words.
Q: Passage: An Italian Post Office opened on September 11, 1923 in Corfu, issuing a set of 8 Italian stamps overprinted "CORFU" which were placed on sale on the 20th. Three additional stamps overprinted in Greek currency arrived on 24th. The third stamp was 2.40 drachma on 1 lire. The Post Office closed at midday on 26 September 1923, only remaining open to dispatch the morning mail. The office had been open for 15 days. Three further values arrived on the day the Post Office closed, and were never issued. They eventually became available for sale at the postal ministry in Rome. Many used copies of these stamps have forged postmarks, but it is known that the Corfu cancel was applied to hundreds of stamps before the Post Office closed.
Question: How many days after the stamps arrived were they placed on sale?
Answer type: Number
A:
9.