Just as I said in subject, when I try to find index of certain element that is in array created of elements with common class name with code: document.getELementsByClassName("blabla").

```js
var a, b, pe;
    a=document.getElementById("a");    
    b=document.getElementById("b");
    pe=document.getElementsByClassName("o");
    var k=pe.indexOf(b);
    alert(k);
```

```html
<p class="o" id="a"></p>
<p class="o" id="b"></p>
```