Don't worry. Here's the formula to retire when you and your wife hit 65.

1. Your 4 room HDB flat is paid up and your children already move out, so you have two empty rooms to rent out.

2. You and your wife have met the CPF min sum. So you get $2,400 pm in total from CPF Life ($1,200 for each person).

3. Each child give $500 pm for you and $500 pm for your wife.

Here's the passive income:

Room rentals from two room (at $1000 pm for each room) = $2,000 pm
CPF Life payout = $2,400 pm
Allowance from children, assuming two children = $2,000 pm
Total passive income = $6,400 pm or $76,800 pa

However this formula is only valid if we continue to welcome FTs so that we can become landlords during our retirement and if you have filial children. If not, then you just sell away your flat and then retire in Malaysia, Thailand or Batam.