input: Write a function to read an array of integers and calculate their sum.
  def calculate_sum(arr):
    sum = 0
    for i in arr:
      sum += i
    return sum
  
  assert calculate_sum([1, 3, 5, 7])==16
  assert calculate_sum([2, 4, 6, 8])==20
  assert calculate_sum([])==0
  assert calculate_sum([-1, -2, -3, -4])==-10
  assert calculate_sum([1, -2, 3, -4])==-2
output: def calculate_sum(arr):
    sum = 0
    for i in arr:
      sum += i
    return sum