MOUSE returns Invalid Input

[Eddie W. Shore]

My goal with the program is to eventually have the user touch one of the "four corners", which each touch turning the a certain color on (from upper left hand: red, green, blue, yellow). I am not all the way there yet. I am having trouble getting input with the MOUSE command - keep getting "Invalid Input" error. And I am stuck to finding where my error is.

Program I have so far:

EXPORT FOURSQUARE()
BEGIN
LOCAL mx, my, L0;
mx=-1; my:=-1;

REPEAT
RECT();
RECT_P(0,0,318,200);
RECT_P(0,201,318,218,0);
TEXTOUT_P("EXIT",130,218,3,
RGB(255,255,255));
WAIT(-1);
L0:=MOUSE();
mx:=B→R(L0(1,1));
my:=B→R(L0(1,2));

// draw boxes here

UNTIL my≥201;
RETURN "DONE";
END;

Map:
(0,0) to (158, 100): red box that lights up with a touch
(159, 0) to (318,100): blue box that lights up with a touch
(0, 101) to (158, 200): yellow box that lights up with a touch
(159, 101) to (318, 200): green box that lights up with a touch
(0, 200) to (218, 318): black exit box. Touch this region to terminate program.

Any help is appreciated.