N = int(input())
S = [''.join(sorted(input())) for _ in range(N)]
d = {}

for s in S:
    for c in range(97, 97+26):
        t = s + chr(c)
        t = ''.join(sorted(t))
        if not t in d:
            d[t] = []
        else:
            d[t].append(s)

for k, v in d.items():
    if len(v) == 0:
        print(k)
        exit()

print(-1)