On April 17, 13 ships arrived at Tokunoshima and dispersed. Two ships arrived at Kanaguma, but nothing happened. Eight ships arrived at Wanya. The ships were besieged all night by 1,000 people. On April 18, Satsuma troops disembarked, fired into the crowds, and killed 50 people. Three ships arrived at Akitoku, and were attacked at the water's edge by the Akitoku people. However, troops quickly fought back and killed 20-30 people. The Satsuma fleet also arrived at Akitoku at 4pm, April 20. On April 21, Kabayama left for Okierabu Island with 10 ships in advance. Others left Tokunoshima at 10am, April 24, and arrived at Okierabu at sunset. They met Kabayama and his ships there, and quickly departed for Ryukyu Island.
Question: How many more ships arrived at their destinations on April 17 than on April 18?
Answer: 20