input: Write a function to read an array of integers and calculate their sum. Return the sum of the array. The input array may be empty in which case return 0.
  assert calculate_sum([1, 3, 5, 7])==16
  assert calculate_sum([-2, 0, 2])==0
  assert calculate_sum([10])==10
  assert calculate_sum([])==0
output: def calculate_sum(arr):
      return sum(arr)