input: Write a python function to measure the execution time of a given function using a decorator.
  assert my_function(10, 20) == 10
  assert my_function(5, 15) == 10
output: import time
  
  def measure_time(func):
      def wrapper(*args, **kwargs):
          start_time = time.time()
          result = func(*args, **kwargs)
          end_time = time.time()
          execution_time = end_time - start_time
          print(f"Execution time: {execution_time} seconds")
          return result
      return wrapper
  
  @measure_time
  def my_function(a, b):
      return b - a