input: Write a python function to calculate the sum of an array.
  assert sum_of_array([3, 5, 6]) == 14
  assert sum_of_array([1, 2, 3, 4, 5]) == 15
  assert sum_of_array([10, 20, 30, 40]) == 100
output: def sum_of_array(arr):
      return sum(arr)