input: Write a python function to calculate the sum of non-negative numbers entered by the user until a negative number is entered.
  assert calculate_sum([10, 20, 30, -1]) == 60
  assert calculate_sum([1, 2, 3, 4, 5, -1]) == 15
  assert calculate_sum([50, 25, 100, -10]) == 175
output: def calculate_sum(numbers):
      total = 0
      for num in numbers:
          if num < 0:
              break
          total += num
      return total