input: Write a function to calculate the sum of an exponential series using the given input value x and the number of terms n. The function should return the sum of the series.
  def exponential_series(x, n):
      sum = 0
      for i in range(n+1):
          sum += x ** i
      return sum
  assert exponential_series(2, 3) == 15
  assert exponential_series(1, 5) == 6
  assert exponential_series(0, 4) == 1
  assert exponential_series(-1, 2) == 1
  assert exponential_series(5, 0) == 1
output: def exponential_series(x, n):
      sum = 0
      for i in range(n+1):
          sum += x ** i
      return sum