How to Inserting a word 'paste' after every five words in a sentence in Msword through c#

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C#

C#4.0

we have a application where we are getting the words written in Msword and splitting the words and comparing the words with the words in the dictionary and coloring them . now we got a requirement that a word paste should be inserted between every 5 words in Msword .Assume below example is a sentence written in MSword .

Example- original text:
this is word used for example. we are showing how the application works...

Example- "paste"inserted text
this is word used for "paste" example. we are showing how "paste"the application works...

like in the above example i have to insert "Paste" for every words.