input: Write a python function to generate a random string of a given length using a specific set of characters.
  assert generate_random_string(8, 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789').isalpha() == False
  assert len(generate_random_string(5, 'abcd')) == 5
  assert set(generate_random_string(10, 'abc')).issubset(set('abc'))
output: import random
  
  def generate_random_string(length, characters):
      return ''.join(random.choice(characters) for _ in range(length))