input: Write a python function to compute the sum of an exponential series.
  assert exponential_series(2, 3) == 15  # 2^0 + 2^1 + 2^2 + 2^3
  assert exponential_series(1, 5) == 6   # 1^0 + 1^1 + 1^2 + 1^3 + 1^4 + 1^5
  assert exponential_series(3, 2) == 13  # 3^0 + 3^1 + 3^2
output: def exponential_series(base, exponent):
      return sum(base ** i for i in range(exponent + 1))