input: Write a function to calculate the sum of two variables using lambda. The lambda function should be defined and then used to print the sum of the given input values.
  assert sum(3, 4) == 7
  assert sum(5, 7) == 12
  assert sum(0, 0) == 0
  assert sum(-1, 1) == 0
  assert sum(-3, -4) == -7
output: sum = lambda x, y: x + y