input: Write a python function to calculate the sum of elements in the array.
  assert sum_of_elements([1, 2, 3, 4, 5]) == 15
  assert sum_of_elements([10, 20, 30, 40, 50]) == 150
  assert sum_of_elements([5, 10, 15]) == 30
output: def sum_of_elements(arr):
      return sum(arr)