Love Calculator javascript

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
  <head>
    <title>Calculator j</title>
    <script>
    var a=0, c=0;
    function erase(){
      if(a==1 && c==0){
        calcola.inserisci.value=' '
        a=0
      }//if
    }//erase
  </script>
  </head>
  <body>
  <form name="calcola">
    <table style="border:7px solid;border-collapse:collapse">
      <tr>
        <td>
          <input type="text" name="inserisci" Size="27px"><br/>
        </td>
      </tr>
      <tr>
        <td>
          <input type="button" value="7" onclick="erase();calcola.inserisci.value+=7">
          <input type="button" value="8" onclick="erase();calcola.inserisci.value+=8">
          <input type="button" value="9" onclick="erase();calcola.inserisci.value+=9">
          <input type="button" value="*" onclick="c=1;calcola.inserisci.value+='*'">
          <input type="button" value="/" onclick="c=1;calcola.inserisci.value+='/'">
          <br/>
          <input type="button" value="4" onclick="erase();calcola.inserisci.value+=4">
          <input type="button" value="5" onclick="erase();calcola.inserisci.value+=5">
          <input type="button" value="6" onclick="erase();calcola.inserisci.value+=6">
          <input type="button" value="+" onclick="c=1;calcola.inserisci.value+='+'">
          <input type="button" value="-" onclick="c=1;calcola.inserisci.value+='-'">
          <br/>
          <input type="button" value="1" onclick="erase();calcola.inserisci.value+=1">
          <input type="button" value="2" onclick="erase();calcola.inserisci.value+=2">
          <input type="button" value="3" onclick="erase();calcola.inserisci.value+=3">
          <input type="button" value=" fflush   " onclick="calcola.inserisci.value=' '">
          <br/>
          <input type="button" value="0" onclick="erase();calcola.inserisci.value+=0">
          <input type="button" value=" .  " onclick="erase();calcola.inserisci.value+='.'">
          <input type="button" value="=" onclick="calcola.inserisci.value=eval(calcola.inserisci.value);a=1;c=0">
        </td>
      </tr>
    </table>
  </form>
  </body>
</html>