Sqlite - update one table based on another table

I have 2 tables T1 (month+year is unique)

month year amount category

• 10 2015 5000 A
• 10 2015 5000 B
• 11 2015 4000 A

T2 month year amount flag

• 10 2015 100 Y
• 10 2015 200 Y
• 10 2015 500 N
• 11 2015 200 Y

I want to update the amount field of T1 as follows for those rows which have category = 'Y' T1.amount = T1.amount - (sum of amount in T2 for matching month,year and having flag = Y)

So after update T1 shall look like this

month year amount category

• 10 2015 4700 A (amount reduced by 300)
• 10 2015 5000 B (no change)
• 11 2015 3800 A (amount reduced by 200)

Kindly help