input: Write a python function to count how frequently the digit 7 appears in numbers less than a specified value which are divisible by either 11 or 13.
  assert count_sevens(50) == 0
  assert count_sevens(78) == 2
  assert count_sevens(79) == 3
  assert count_sevens(100) == 3
  assert count_sevens(200) == 6
  assert count_sevens(4000) == 192
  assert count_sevens(10000) == 639
  assert count_sevens(100000) == 8026
output: def count_sevens(n):
      count = 0
      for i in range(n):
          if i % 11 == 0 or i % 13 == 0:
              count += str(i).count('7')
      return count