input: Write a function to calculate an exponential series. The function should take two parameters, x and n, and return the sum of the first n terms of the exponential series of x.
  assert exponential_series(2, 3) == 15
  assert exponential_series(1, 4) == 5
  assert exponential_series(0, 5) == 1
  assert exponential_series(3, 0) == 1
output: def exponential_series(x, n):
      result = 0
      for i in range(n):
          result += x ** i / math.factorial(i)
      return result