from collections import Counter
from bisect import bisect_left, bisect_right, insort


def ch_gen(start="a", stop="z"):
   for i in range(ord(start), ord(stop) + 1):
      yield chr(i)


ABC = [*ch_gen()]

N = int(input())
S = []
for _ in range(N):
   s = sorted(input())
   S.append(s)

C = Counter()
for s in S:
   for ch in ABC:
      insort(s, ch)
      u = "".join(s)
      C[u] += 1
      s.remove(ch)

for s in S:
   pool = []
   for ch in ABC:
      insort(s, ch)
      u = "".join(s)
      pool.append(u)
      s.remove(ch)
   for u in pool:
      C[u] -= 1
      if C[u] == 0:
         del C[u]
   for u in pool:
      if C[u] == 0:
         print(u)
         exit()
   for u in pool:
      C[u] += 1

print(-1)