calculate the quantity of each size of an item

Hello!
I need some help please ....I need a formula that will calculate the quantity of each size of an item (XS,S,M,L,XL). For example to calculate the quantity for Mediums, I need the formula to look at column C, if M is entered, then look at the adjacent cell in column A for the quantity and formulate a total number (from rows 4-21) and enter the total quantity in C27.
See attachment.
Thanks for your help! Kim