n = int(input())
cnt = {}
for _ in range(n):
    S = input()
    for i in range(26):
        T = "".join(sorted(S + chr(97 + i)))
        cnt[T] = cnt.get(T, 0) + 1

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