[Solved] Count column C where column D = "A"?

[Kirjava]

I already tried AI, it couldn't figure this out and all of their suggestions returned ERR:508.
I don't know if what I'm asking about is even possible.
List of films in storage. I want to check if there's any space left in each box without pulling it out.
The boxes fit 2 rows of 21 DVDs, so I split it so Box-1 = Row-A + Row-B, Box-2 = Row-C + Row-D, etc

List of films, tv series, and games:
A_______B______C____D
Title__Format__Size__Row
Item-1__DVD_____8____A
Item-2__DVD_____4____B
Item-3__BRD_____1____C
Item-4__UMD____.5____C
The size is how big the box sets are, meaured in regular DVD cases.

(hidden...)
_______E_______________F________ etc.
=IF(D#="A";C#;0)__=IF(D#="B";C#;0)__ etc.

AH___AI
Box__Total
A____=SUM(E2:E#)
B____=SUM(F2:F#)
C____=SUM(G2:G#)
etc.

I want it to count the total in column C where column D = "A" without having a hidden column for it.
Once I figure it out I'll probably think of a dozen uses for it.

[MrProgrammer]

I want it to count the total in column C where column D = "A"

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[Kirjava]

Thank you. I knew there should be an easier way to do it.