import sys, time, random
from collections import deque, Counter, defaultdict
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
inf = 2 ** 61 - 1
mod = 998244353
import string
n = ii()
s = [sorted(input()) for _ in range(n)]

for v in s:
    if s.count(v) == 1:
        for x in string.ascii_lowercase:
            if sorted(''.join(v) + x)[:-1] not in s:
                print(''.join(v) + x)
                exit()
print(-1)