A series with same common difference is known as arithmetic series. The first term of series is 'a' and common difference is d. The series looks like a, a + d, a + 2d, a + 3d, . . . Find the sum of series upto nth term.

Example 1:
Input: n = 5, a = 1, d = 3
Output: 35
Explanation: Series upto 5th term is
1 4 7 10 13, so sum will be 35.
Example 2:
Input: n = 3, a = 1, d = 2
Output: 9
Example: Series upto 3rd term is
1 3 5, so sum will be 9.

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

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

Constranits:
1 <= n, a, d <= 10000

见题面

见题面