n = int(input())
s = [input() for _ in range(n)]

m = min(n, 10)
used = [False for _ in range(1 << m)]
for i in range(n):
  mask = 0
  for j in range(m):
    if s[i][j] == 'b':
      mask |= (1 << j)

  used[mask] = True

for mask in range(1 << m):
  if not used[mask]:
    ans = []
    for j in range(m):
      if (mask >> j) & 1 != 0:
        ans.append('b')
      else:
        ans.append('a')

    if m < n:
      ans.append("a" * (n - m))

    print(*ans, sep="")
    exit()