input: Write a function to perform vector addition, subtraction, and scalar multiplication.
  assert (Vector2D(1, 2) + Vector2D(3, 4)).x == 4 and (Vector2D(1, 2) + Vector2D(3, 4)).y == 6
  assert (Vector2D(5, 6) - Vector2D(2, 3)).x == 3 and (Vector2D(5, 6) - Vector2D(2, 3)).y == 3
  assert (Vector2D(2, 3) * 4).x == 8 and (Vector2D(2, 3) * 4).y == 12
output: class Vector2D:
      def __init__(self, x, y):
          self.x = x
          self.y = y
  
      def __add__(self, other):
          return Vector2D(self.x + other.x, self.y + other.y)
  
      def __sub__(self, other):
          return Vector2D(self.x - other.x, self.y - other.y)
  
      def __mul__(self, scalar):
          return Vector2D(self.x * scalar, self.y * scalar)