Editfelder für Buchstaben sperren

[naudoc]

Hallo,
was ist hier Falsch:

Delphi-Quellcode:

procedure TForm1.Edit1Change(Sender: TObject);
var a,b,c:real;
begin
if ((edit1.text=floattostr(a)) or (edit1.text='-'+floattostr(a)+'') or (edit1.text='0') or (edit1.text=''))
and ((edit2.text=floattostr(b)) or (edit2.text='-'+floattostr(b)+'') or (edit2.text='0') or (edit2.text=''))
and ((edit3.text=floattostr(c)) or (edit3.text='-'+floattostr(c)+'') or (edit3.text='0') or (edit3.text=''))
then

mfG naudoc