input: Write a function to calculate the sum of a geometric series.
  assert find_sum_series(1, 2, 3) == 7
  assert find_sum_series(3, 2, 4) == 45
  assert find_sum_series(2, 3, 3) == 26
output: def find_sum_series(a, r, n):
      return a * (1 - r**n) // (1 - r)