如何从所有元素中提取“失败”并将它们相加?
<gateway>
<smscs>
<count>3</count>
<smsc>
<id>a</id>
<received><sms>0</sms><dlr>0</dlr></received>
<sent><sms>10537</sms><dlr>0</dlr></sent>
<failed>13</failed>
<queued>6272</queued>
</smsc>
<smsc>
<id>b</id>
<received><sms>0</sms><dlr>0</dlr></received>
<sent><sms>10530</sms><dlr>0</dlr></sent>
<failed>10</failed>
<queued>6284</queued>
</smsc>
<smsc>
<id>c</id>
<received><sms>0</sms><dlr>0</dlr></received>
<sent><sms>10679</sms><dlr>0</dlr></sent>
<failed>11</failed>
<queued>6291</queued>
</smsc>
</smscs>
</gateway>