input: Write a python function to check if a number exists in a set of random numbers generated from 1 to 1000.
  assert check_if_exists(10) in [True, False]
  assert check_if_exists(500) in [True, False]
  assert check_if_exists(1001) == False
output: import random
  
  def check_if_exists(number):
      random_set = set(random.randint(1, 1000) for _ in range(1000))
      return number in random_set if 1 <= number <= 1000 else False