def add_edge(node, adj_lst, adj_rev, s1, s2):
  ind = 0
  max_len = min(len(s1), len(s2))
  while ind < max_len and s1[ind] == s2[ind]:
    ind += 1
  if ind == max_len:
    if max_len < len(s1):
      return True
    return False
  c1 = ord(s1[ind]) - ord("a")
  c2 = ord(s2[ind]) - ord("a")
  adj_lst[c1].add(c2)
  adj_rev[c2].add(c1)
  node.add(c1)
  node.add(c2)
  return False
"""
def dfs(x, visited, adj_lst, order):
  visited[x] = True
  for to in adj_lst[x]:
    if not visited[to]:
      dfs(to, visited, adj_lst, order)
  order.append(x)

def dfs_rev(x, used, adj_rev):
  used.append(x)
  for to in adj_rev[x]:
    if not to in used:
      dfs_rev(x, used, adj_rev)
"""
while True:
  n = int(input())
  if n == 0:
    break
  
  lst = [input() for _ in range(n)]
  node = set()
  adj_lst = [set() for _ in range(26)]
  adj_rev = [set() for _ in range(26)]
  brunk_flag = False
  for i in range(n):
    for j in range(i + 1, n):
      brunk_flag = brunk_flag or add_edge(node, adj_lst, adj_rev, lst[i], lst[j])
  
  L = []
  visited = [False] * 26
  cycle_flag = False
  def visit(n):
    global cycle_flag
    if cycle_flag: return
    if visited[n] == 2:
      cycle_flag = True
    elif visited[n] == 0:
      visited[n] = 2
      for to in adj_lst[n]:
        visit(to)
      visited[n] = 1
      L.append(n)
  
  L = []
  for n in node:
    visit(n)
  if cycle_flag or brunk_flag:
    print("no")
  else:
    print("yes")
