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 first overprinted stamps were placed on sale did the second set of overprinted stamps arrive?
Answer: 4