Oh God, the nay-saying. Take all these "I dunno"'s elsewhere, and don't convert them to "it can't be done"'s. It's that simple.

:ClrHome

:For(S,0,1

:"(→Str1

:For(T,0,1

:Repeat K=105

:Output(S+1,1,Str1+sub(",)",T+1,2-T)+"<12 spaces>

:Repeat Ans or K=45 or 2>abs(5-abs(5-abs(K-83

:getKey→K

:Ans>E2

:End

:round(13fPart(13‾¹(K-Ans

:If Ans and 7T+7≠length(Str1

:Str1+sub("1234567890.‾",Ans,1→Str1

:If K=45

:DelVar T"(→Str1

:End

:If not(T

:Str1+",→Str1

:End

:expr("{"+sub(Str1,2,length(Str1)-1→L2

:If not(S

:Ans→L1

:End

:ClrHome

:Disp L1,L2

L1 and L2 are stored rather than A/B/X/Y. This supports decimal points and negation, and is limited to up to 6 characters per *x*, and up to 12-length(*x*) characters per *y*. CLEAR erases progress for the entire coordinate. A syntax error and 2:Goto will divert to the homescreen instead of the editor. Really though, if you consider the sequence of key presses for these inputs, it's *identical* to the following:

:Input "x1=",X

:Input "y1=",Y

:Input "x2=",A

:Input "y2=",B

Ask yourself whether bells and whistles is worth hindering efforts to solve the problem.