Sum a value in Entity Collection in C#

Can someone help me figure out how to sum the quantity value here in the EntityCollection in C#?

Thanks in advance.

  string fetchXmlResult =
                             @"<fetch version='1.0' output-format='xml-platform' mapping='logical' distinct='false' >
  <entity name='salesorder'> 
                                                 <attribute name='productid'/>
                                                <attribute name='ordernumber'/> 
                                                <attribute name='quantity'/>  
  </entity>
</fetch>";

XmlElement element1= xml.CreateElement(string.Empty, "Order", xml.DocumentElement.NamespaceURI);
           

       EntityCollection xmlResult = _crmservice.RetrieveMultiple(new FetchExpression(fetchXmlResult())); 
 foreach (Entity e in xmlResult.Entities)
                {


                    XmlElement ordernumber= xml.CreateElement("OrderNumber", xml.DocumentElement.NamespaceURI);
                    ordernumber.InnerText =  e.Attributes["ordernumber"]).ToString();
                    element1.AppendChild(ordernumber);

                    XmlElement productid= xml.CreateElement("ProductId", xml.DocumentElement.NamespaceURI);
                    productid.InnerText =  e.Attributes["productid"]).ToString();
                    element1.AppendChild(productid);
}

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