How do I sort a xelement in C# based on it's child values?

<Articles>
<Article id="article_1-1" file="article_1-1.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_1-3" file="article_1-3.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_4-1" file="article_4-1.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_4-2" file="article_4-2.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_4-3" file="article_4-3.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_5-1" file="article_5-1.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_1-2" file="article_1-2.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_2-1" file="article_2-1.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_2-2" file="article_2-2.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_1-4" file="article_1-4.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_13-1" file="article_13-1.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_6-1" file="article_6-1.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_6-2" file="article_6-2.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_6-3" file="article_6-3.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_10-1" file="article_10-1.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_10-2" file="article_10-2.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_11-1" file="article_11-1.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_11-2" file="article_11-2.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_14-1" file="article_14-1.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_14-2" file="article_14-2.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_15-1" file="article_15-1.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_15-2" file="article_15-2.xml">
<title ref="headline">Title value</title>
</Article>
<Article id="article_16-1" file="article_16-1.xml">
<title ref="headline">Title value</title>
</Article>
</Articles>

Quote:

System.ArgumentOutOfRangeException: 'Index and length must refer to a location within the string.
Parameter name: length'

xrepdoc.Root.Element(replicaNamespace + "Articles").Elements(replicaNamespace + "Article")
.OrderBy(el => Int32.Parse(el.Attribute("id").Value.Substring(el.Attribute("id").Value.IndexOf("_"), el.Attribute("id").Value.IndexOf("-"))))
.ThenBy(el => Int32.Parse(el.Attribute("id").Value.Substring(el.Attribute("id").Value.IndexOf("-") + 1)))
.ToList();