[USER (data scientist)]: Thanks! Now, I'd like to calculate the percentage change in the annual number of affordable properties sold and the percentage change in the average price of affordable properties. Please generate a Series for both the annual number and average price of affordable properties, showing their percentage changes, and store them in pickle files.
My template of code snippet is:

---BEGIN CODE TEMPLATE---

import pandas as pd
import pickle

melbourne_housing = pd.read_csv("melb_data.csv") 

# YOUR SOLUTION BEGIN:
<code1>
[GENERATE YOUR CODE]
</code1>
# YOUR SOLUTION END:

print(percentage_change_annual_number)

# save data
pickle.dump(percentage_change_annual_number,open("./pred_result/percentage_change_annual_number.pkl","wb"))

print(percentage_change_average_price)

# save data
pickle.dump(percentage_change_average_price,open("./pred_result/percentage_change_average_price.pkl","wb"))

---END CODE TEMPLATE---

[YOU (AI assistant)]: You can use the pct_change() method to calculate the percentage change for both metrics. Here's the code: 
