pascal. перечислительный тип

program Cells;
uses Crt;

type
 BOOL = (FALSE, TRUE);

 function CheckCells(x1, y1, x2, y2: integer): BOOL;
 begin
  if (x2 - x1 = 0) then
   CheckCells := FALSE
  else
   if (Abs((y2 - y1) / (x2 - x1)) = 1) then CheckCells := TRUE else CheckCells := FALSE;
 end;

var
 x1, y1, x2, y2: integer;

begin
 ClrScr;
 WriteLn('Enter x1, y1, x2, y2: ');
 ReadLn(x1, y1, x2, y2);
 if (CheckCells(x1, y1, x2, y2) = TRUE) then WriteLn('YES') else WriteLn('NO');
 ReadLn;
end.