3 level matching in xslt

My input xml looks like as shown below `

<a>  
 <b>
  <b1>ID1</b1> 
  <b2>text1</b2>
  <b3>c10</b3>
  <b3>c20</b3>
 </b>
  <b>
  <b1>ID2</b1> 
  <b2>text2</b2>
  <b3>c10</b3>
 </b>
  <b>
  <b1>ID3</b1> 
  <b2>text3</b2>
  <b3>c20</b3>
 </b>
  <c>
   <c1>c10</c1>
   <c2>text123<c2>
   <c3>d10</c3>
  </c>
  <c>
   <c1>c20</c1>
   <c2>text123<c2>
   <c3>d20</c3>
   </c>
   <d>
    <d1>d10</d1>
    <d2>text11</d2>
    <d3>10</d3>
   </d>
   <d>
    <d1>d20</d1>
    <d2>text33</d2>
    <d3>20</d3> 
   </d>
</a>

I want to add the for the corresponding value that comes inside corresponding which finally falls under . The Output should look like

<b>
 <b1>ID1</b1>
 <b4>30</b4>
<b>
<b>
 <b1>ID2</b1>
 <b4>10</b4>
<b>
<b>
 <b1>ID3</b1>
 <b4>20</b4>
<b>

`

Quick Response will be highly appreciated.

Regards Ayush