from collections import defaultdict
N=int(input())
S=[]
dic=defaultdict(int)
abc='abcdefghijklnmopqrstuvwxyz'
for _ in range(N):
    S.append(list(input()))
    S[-1].sort()
    for i in range(len(abc)):
        dic[tuple(sorted(S[-1]+[abc[i]]))]+=1
for k in dic.keys():
    if dic[k]==1:
        print(''.join(k))
        exit(0)
print(-1)