Salary statement - Help with salary increase

[sakkthi]

Salary increments will be given once in a year at random times as fixed amount.

Increments are given randomly, so if 5000 rs increment is given this month, it should be added to allowances (increases to 14500), so it increases gross salary to 40,000, but next month there won't be any increment, but the 5000 added to allowances should be retained there, even if we put ZERO in the increment cell for this month

https://docs.google.com/spreadsheets/d/1eGWm0Qn7bxGb6qYprsyW6wZWRUjjbhQJLihbNF7oXo0/edit?usp=sharing

Thanks