TI 89 program missing a parenthesis

breebreebran
I'm trying to write a program to find midpoint and distance.
And here's what I have.

Code:
Prompt dist
If dist=1 then
Prompt x,y,x2,y2
√((x2-x)^2+(y2-y)^2)→d
Disp d
Else
Prompt xx1,xx2,yy1,yy2
(xx1+xx2)/2→a
(yy1+yy2)/2→b
Disp a,b
Endif
Endprgm

When I try to run this program it says I am missing a parenthesis somewhere.
And I had modified the code before. I don't remember how. But when I did that it said that the variable was already reserved or something like that.