import sys
sys.setrecursionlimit(5*10**5)
input = sys.stdin.readline
from collections import defaultdict, deque, Counter
from heapq import heappop, heappush
from bisect import bisect_left, bisect_right
from math import gcd

def sti(s):
    res = []
    for c in s:
        res.append(ord(c) - ord('a'))
    return res

def its(s):
    res = []
    for i in s:
        res.append(chr(i + ord('a')))
    return res

n = int(input())
s = []
d = defaultdict(int)
for i in range(n):
    si = sti(input().rstrip())
    si.sort()
    s.append(si)
    d["".join(its(si))] += 1

for i in range(n):
    for j in range(26):
        new = s[i] + [j]
        new.sort()
        cnt = 0
        for k in range(len(new)):
            tmp = new[:k] + new[k+1:]
            cnt += d["".join(its(tmp))]
        if cnt == 1:
            print("".join(its(new)))
            exit()
print(-1)