Is there a way to count the number of occurrences of a category and assign a value in Excel?

Suppose there's a table with the following characteristics:

Subject    Topic     Issue  Subject RAG Rating
Biology    Cells     Y      
Biology    Mitosis   Y      
Maths      Algebra   N      
Computing  Python    N      
Computing  R         Y      

Where Subject RAG Rating is a blank column. I want to populate this column with either R if the subject has two or more issues; A if the subject has 1 issue; or G if the subject has no issues.

Is there a formula that can do this and return a value for each row that looks like:

Subject    Topic     Issue  Subject RAG Rating
Biology    Cells     Y      R
Biology    Mitosis   Y      R
Maths      Algebra   N      G
Computing  Python    N      A
Computing  R         Y      A

I thought that perhaps COUNTIF might work, i.e. count the number of Y or N, but I'm unsure how you can tell Excel to populate the rows with R, A or G. Any help appreciated!