Letters to You (I'm not sure this is a good idea) #JustWriteIt

7. Come on Mathea, have you ever watched Gravity Falls? / I Suck

You come to school a week later to find a piece of paper neatly folded and placed on top of your desk. You take it and find it is a printed message...

... Written in comic sans.
You cringe at the sight of the font, but read the note anyway.
"13-18 1-20-2-1-19-8 9-19 19-1-4 20-8-1-20 25-15-21 4-9-4-14-20 12-9-11-5 8-9-13
13-18 3-1-5-19-1-18-9-1-14 23-9-12-12 19-21-2-19-20-9-20-21-20-5"
They have to be fucking kidding you.
You look at the back of the piece of paper and notice that there is a Post Script written in perfect cursive, faint pencil traces adress you specifically in the part that was sitting against the table.
"P.S: Come on, Mathea, haven't you ever watched Gravity Falls?"
Gravity Falls? What does this have to do with Gravity fricking Falls?
Oh.
Oh.
You whip out your phone and type a URL from memory.
rumkin.com/tools/cipher
After skimming through the list for the right code, you tap on "numbers" and copy digita and dashes into your phone. The text that appear when you tap "decrypt" leaves you speechless.
"MR ATBASH IS SAD THAT YOU DIDNT LIKE HIM
MR CAESARIAN WILL SUBSTITUTE"
Why are you so fucking dense?
The message is in the same format as that one encrypted announcement for code change in Gravity Falls season 1.
You need to get that other note back and decrypt it, read the letter encoded in the reversed alphabet cipher.

***

..Hi, so I know no one is reading this anyway, but I still suck.
I didn't finish this in time, which was yesterday (The last day of August), So I completely failed the #JustWriteIt challenge, heh.
Buttttt I'm still finishing this story because I have it planned and I like it, so maybe don't expect to see it in little spurs of creativity since it's too long, but I'm going to finish it. Yeah, just that thing. I'm sorry.