from collections import defaultdict


res = defaultdict(int)
for _ in range(int(input())):
    s = input().strip()
    for i in range(26):
        nc = chr(ord('a')+i)
        res[''.join( sorted(s+nc)) ]+=1
for i,v in res.items():
    if v==1:
        print(i)
        exit()
print(-1)