Given n, a and r as the number of terms, first term and common ratio respectively of an Geometric Progression Series. Find the sum of the series upto nth term.

Example 1:
Input: 3 3 2
Output: 21
Explanation: Series upto 3td term is
3 6 12, so sum will be 21.
Example 2:
Input: 3 1 2
Output: 7
Example: Series upto 3rd term is
1 2 4, so sum will be 7.

Your Task:
You don't need to read or print anything. Your task is to complete the function sum_of_gp() which takes n, a and r as input parameter and returns the sum of the series.

Expected Time Complexity: O(1)
Expected Space Complexity: O(1)

Constranits:
1 <= n, a, r <= 10

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