[F8] invalidate textfield?

[sonic88]

Hello
I have no idea how to do this in AS2:
I have four text entry fields, txt1, txt2, txt3 and txt4.
To operate with them I can only combine (txt1 and txt2) or (text3 and text4), but not mixing for example txt1 and txt4.
How can invalidate txt3 and 4 if the user introduced a number in text1?
any suggestions?
Thank you in advance

[calmchess]

btn0.onPress = function(){
if (txt1.text != "" && text2.text != "" ){
txt3.text = "";
txt4.text = "";
out0.text = txt1.text += txt2.text;
}
}

btn1.onPress = function(){
if (txt3.text != "" && text4.text != "" ){
txt1.text = "";
txt2.text = "";
out1.text = txt3.text += txt4.text;
}
}

[sonic88]

Thank you so much, calmchess.