input: Write a function to add two numbers together and print the result. Define the function to take two arguments and return the sum of the two numbers.
  assert add(5, 7)==12
  assert add(0, 0)==0
  assert add(-3, 3)==0
  assert add(10, -5)==5
  assert add(-1, -1)==-2
output: def add(a, b):
      return a + b