Calculating Lag Period of 2 Months

Essentially Row 11 comes from a Rental calculation sheet and it shows the accummulated no. of rooms we are renting in a particular month
In Month 0 the 3 is rooms is owned by us to its always there and can be rented out immediately
But for the rooms we are renting if we want to sub lease these rooms, we think it will take at least 2 months to sublease so a lag period of 2 month
So Row 13 is the Row I want to see the number as in the example (sheet)--- hardcoded
For examples,

Month 0 - 3 rooms
Month 1- 3 rooms
Month 2- 3 rooms
Month 3- 5 rooms (2 rooms added from the rooms which were rented in Month 1)


With Row 13 I can calculate my revenue and expenses so its more important to me.

I tried various ways but could not come up with a formula to do so. I would really appreciate if any help on this.