Given two identical DataFrames Sales16 and Sales17. But Sales17 has some values missing. Write code so that Sales17 fills its missing values from corresponding entries of Sales16.

import pandas as pd
data_sales16 = {
    'Product': ['A', 'B', 'C', 'D'],
    'Sales': [100, 150, 120, 180]
}
Sales16 = pd.DataFrame(data_sales16)
data_sales17 = {
    'Product': ['A', 'B', 'C', 'D'],
    'Sales': [100, None, 120, None]
}
Sales17 = pd.DataFrame(data_sales17)
Sales17 = Sales16.fillna({'B': 150, 'D': 180})
print("Sales16:")
print(Sales16)
print("\nSales17 (after filling missing values):")
print(Sales17)

Sales16:
  Product  Sales
0       A    100
1       B    150
2       C    120
3       D    180

Sales17 (after filling missing values):
  Product  Sales
0       A    100
1       B    150
2       C    120
3       D    180