How to auto fill input form with JavaScript

i have some problem, in my case, i have 2 form

<form>
<input type="text" name="a" id="fill" />
<select name="b" id="status">
<option value="success">Success</option>
<option value="fail">fail</option>
</select>
</form>

If the content of word A is more than 150, select B is success. If the content of word a is less than 150, select B is fail.

i don't know script java script for my problem, please help me