import collections,sys,math,functools,operator,itertools,bisect,heapq,decimal,string,time,random
#sys.setrecursionlimit(10**9)
#sys.set_int_max_str_digits(0)
#input = sys.stdin.readline
n = int(input())
se = [set() for i in range(n)]
a = []
for i in range(n):
    check = 0
    s = input().rstrip()
    a.append(s)
    for j in s:
        check += pow(12,ord(j) - ord('a'))
    for j in range(26):
        check += pow(12,j)
        se[i].add(check)
        check -= pow(12,j)

for i in range(n):
    for j in se[i]:
        cnt = 0
        for k in range(n):
            if k == i:
                continue
            if len(a[i]) != len(a[k]):
                continue
            if j in se[k]:
                cnt += 1
        if cnt == 0:
            for k in range(26):
                for l in range((j % pow(12,k+1)) // pow(12,k)):
                    print(chr(ord('a') + k),end='')
            print()
            exit()
print(-1)