input: Write a python function to add and subtract complex numbers using the given ComplexNumber class.
  assert str(ComplexNumber(1, 2) + ComplexNumber(3, 4)) == '4 + 6i'
  assert str(ComplexNumber(5, 7) - ComplexNumber(2, 3)) == '3 + 4i'
  assert str(ComplexNumber(0, 0) + ComplexNumber(0, 0)) == '0 + 0i'
  assert str(ComplexNumber(-1, -1) - ComplexNumber(1, 1)) == '-2 + -2i'
output: class ComplexNumber:
      def __init__(self, real, imag):
          self.real = real
          self.imag = imag
  
      def __add__(self, other):
          return ComplexNumber(self.real + other.real, self.imag + other.imag)
  
      def __sub__(self, other):
          return ComplexNumber(self.real - other.real, self.imag - other.imag)
  
      def __str__(self):
          return f'{self.real} + {self.imag}i'