from collections import Counter
from string import ascii_lowercase

cnt = Counter()
for _ in range(int(input())):
  S = input()
  for c in ascii_lowercase:
    T = "".join(sorted(S + c))
    cnt[T] += 1

for T, n in cnt.items():
  if n == 1:
    print(T)
    break
else:
  print(-1)