Optimise partial sum calculation

I want to calculate partial sums for different parameters a, n:

I also know that all partial sums I will need to calculate are less or equal to 10¹⁸.

I tried the code with the sum calculation formula:

if __name__ == '__main__': n, a = map(int, input().split()) if a != 1: a_n1 = a ** (n + 1) res = (n * a ** (n + 2) - a ** (n + 1 * (n + 1) + a) // (a - 1) ** 2 else: res = sum([i for i in range(n + 1)]) print(round(res))

however it increases the memory limit of 256 Mb (the straight sum calclulation increases a time limit of 1sec).

Could you please help me optimise the code in python, so it takes less memory (I don't have an option of using numpy, etc.)

Thanks