new_contents: def solution(rocks):
      elems = set(rocks[0])
      
      for index in range(1, len(rocks)):
          rock = set(rocks[index])
          
          for elem in tuple(elems):
              if elem not in rock:
                  elems.remove(elem)
                  
      return len(elems)
  
  count = int(input())
  rocks = tuple(input().strip() for i in range(count))
  
  print(solution(rocks))
  
subject: Create solution.py