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([-1, -3, 5, 7])==8
  assert calculate_sum([0, 0, 0, 0])==0
  assert calculate_sum([])==0
  assert calculate_sum([1])==1
output: def calculate_sum(arr):
    sum = 0
    for i in arr:
      sum += i
    return sum