Making letters compute as numbers then averaging them

Hello all. I am currently an Academy Director at a school and I am trying to develop a skills tracker that is user friendly for my staff. I want them to be able to put in an M that equals 100, S that equals 70 and N that equals 50 so I came up with the following formula
=COUNTIF(D12:Y12,"M")*100+COUNTIF(D13:Y13,"S")*70+COUNTIF(D13:Y13,"N")*50

However, I need the average of these numbers as well. For instances, teachers will go down row 1 (A-M) and place an M, S, or N. IN N1 i would like the average of those input. Please help