Ok, I will take a guess at the logic rules. How about:
string newStr = str.Substring(0,4)+str.Substring(10,4)+str.Substring(15,6);
or maybe you want
string newStr = str.Substring(0,4)+str.Substring(10,4)+str.Split("-")[1];
there are many things you could want
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solved How to truncate string value in c#? [closed]