pi amortization

I am needing to write a program that calculates the amortization of pi. The output must look like this:
This program approximates pi using an n-term series expansion.
Enter the value of n> -3
-3 is an invalid number of terms.

or if it is a positive number that can actually be used for calculation:
This program approximates pi using an n-term series expansion.
Enter the value of n> 6
pi[6] = 4[1 - 1/3 + 1/5 - 1/7 + 1/9 - 1/11] = 2.97604617604617560644

I understand enough about c++ to know what to use, I just don't know it good enough to write the code. I understand that there will probably need to be a void function in order to calculate the amortization. Like I said, I understand the nuances and how they work I just can't apply them to write a successful c++ program. If anyone could help me with the more difficult parts of the program, like the calculation and correct print format that would be AWESOME!! Thank you for your time.
Thanks for the link. I think I opened about every one but that one haha.
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