Try to make a question with variabels

Hello I'm new in this forum but i hope you understand what I trying to do.
 
 

<?php
$theanswer = 4;
$answer = $_POST['answer'];

if(isset($_POST['button'])){

if ($theanswer = $answer){
echo "Right the answer is 4<br>";

} else {

echo "Wrong try again!";

}


}
?>
2 + 2 is?
<form method="post" action="new.php">
<input type="text" name="answer">
<input type="submit" name="button">
</form>

 
 
The code don't work and I get an notice too.
 
Notice: Undefined index: answer in C:\wamp\www\ss\new.php on line 4
 
I think you understand what i want but i will explain if you don't.
 
If they write 4 in the textfield the echo will say Right the answer is 4. But if they write anything else
it will say wrong try again!

Now i wonder what have i do wrong?