Task: Question Answering
Input (English): Passage: After their success in the Morea, the Ottomans moved against the Venetian-held Ionian Islands. They occupied the island of Lefkada , which the Venetians had taken in 1684, and the fort of Butrinto opposite the city of Corfu. On 8 July 1716, an Ottoman army of 33,000 men landed on Corfu, the most important of the Ionian islands. Despite an indecisive naval battle on the same day, the Ottoman land army continued its disembarkment and advanced towards the city of Corfu. On 19 July, after capturing the outlying forts of Mantouki, Garitsa, Avrami and of the Saviour, the siege began. The defence was led by Count Johann Matthias von der Schulenburg, who had roughly 8,000 men at his command. The extensive fortifications and the determination of the defenders withstood several assaults. After a great storm on 9 August—which the defenders attributed to the intervention of Corfu's patron saint, Saint Spyridon—caused significant casualties among the besiegers, the siege was broken off on 11 August and the last Ottoman forces withdrew on 20 August. 
Question: How many days after the siege was broken off did the Ottoman forces withdraw? 
Answer type: Number 
Output (English): 9.