input: Write a python function to generate a random string of specified length using a given set of characters.
  assert len(generate_random_string(10)) == 10
  assert len(generate_random_string(20)) == 20
  assert len(generate_random_string(5)) == 5
output: import random
  
  def generate_random_string(length, characters='abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'):
      return ''.join(random.choice(characters) for _ in range(length))