I'm doing my math game and I have problem with char(plus or minus). How can I do it to plus or minus be just before the number? I need to check entered number, I don't want situation for example "--8-88". Problem is in input button with name "minus". 

```
<body onload="main_page();">
<form name="game">
    <div style="float: left;" class="screen">
        <input type="text" id="answer" value="" autofocus()>
    </div>
    <input type="button" value="-" onclick="game.answer.value += '-'" name="minus">
    <input type="button" value="8" name="eight" onclick="game.answer.value += '8'" >
</form>

```