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)]
ts = []
for v in s:
    for x in string.ascii_lowercase:
        ts.append(sorted(v + [x]))
        
for v in ts:
    # v から1文字抜いたものがsに含まれているか
    cnt = 0
    for i in range(len(v)):
        cnt += s.count(v[:i] + v[i+1:])
    if cnt == 1:
        print(''.join(v))
        exit()
print(-1)