input: Write a python function to find the index where the sum of elements to the left equals the sum of elements to the right.
  assert index_sum_equal([1, 7, 3, 6, 5, 6]) == 3
  assert index_sum_equal([1, 2, 3, 4, 5, 6]) == -1
  assert index_sum_equal([2, 1, -1, 2, -1, 1]) == 0
output: def index_sum_equal(nums):
      total_sum = sum(nums)
      left_sum = 0
      for i, num in enumerate(nums):
          if left_sum == (total_sum - left_sum - num):
              return i
          left_sum += num
      return -1