Assuming your string will always have only two -s, you could using the following to get the substring between them. If this is not the case please modify the question to better describe the issue.
string myString = AccountName.Split('-')[1];
Check out https://msdn.microsoft.com/en-us/library/system.string.split(v=vs.110).aspx for more information on the Split method in the string class.
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solved Select a part in a string C# [closed]