from collections import defaultdict
dic = defaultdict(int)
N = int(input())
for _ in range(N):
    s = list(input())
    for i in range(26):
        dic["".join(sorted(s+[chr(97+i)]))] += 1
for k,v in dic.items():
    if v == 1:
        print(k)
        exit()
print(-1)