from collections import defaultdict, Counter
from string import ascii_lowercase

N = int(input())
S = [''.join(sorted(input())) for _ in range(N)]

if N == 1:
    print(S[0] + 'z')
    exit()

di = defaultdict(list)

for c in ascii_lowercase:
    for s in S:
        di[c].append(''.join(sorted(s + c)))

for c in ascii_lowercase:
    for key, val in Counter(di[c]).items():
        if val == 1:
            print(key)
            exit()

print(-1)