input: Write a function to print prime numbers up to a given number.
  assert print_prime_numbers(10) == "2357"
  assert print_prime_numbers(20) == "235711131719"
  assert print_prime_numbers(5) == "235"
output: def print_prime_numbers(n):
      def is_prime(num):
          if num <= 1:
              return False
          for i in range(2, int(num**0.5) + 1):
              if num % i == 0:
                  return False
          return True
  
      prime_numbers = ""
      for i in range(2, n + 1):
          if is_prime(i):
              prime_numbers += str(i)
      
      return prime_numbers