XML : need help - XML in ASP.Net

i need help in ASP.Net to do the below. Hope someone could hint me some direction.

a. ASP.Net web service to receive the XML document via third party URL
b. Validate the XML, if any other format than the example below structure, it will be considered invalid.
c. Once validated, it will retrieve the information and summarize the total number of "ShippedQty" and return the value.

      <?xml version="1.0" encoding="utf-8" standalone="yes"?>  <WMS xmlns="http://blog.cripperz.sg">    <Order>      <Header>        <RARefNum>RASO000001</RARefNum>        <RARefNum>RASO000001</RARefNum>        <WMSCategory>ESSVMI</WMSCategory>        <CustomerID>nVidianVidia</CustomerID>        <ShippedQty>10</ShippedQty>      </Header>      <Header>        <RARefNum>RASO020001</RARefNum>        <RARefNum>RASO0020001</RARefNum>        <WMSCategory>ESSVMI</WMSCategory>        <CustomerID>ascent</CustomerID>        <ShippedQty>20</ShippedQty>      </Header>    </Order>  </WMS>