N = int(input())

dct = dict()
for _ in range(N):
    s = list(input())
    for i in range(26):
        t = s[:]
        t.append(chr(i+ord("a")))
        t.sort()
        t = "".join(t)
        if t not in dct:
            dct[t] = 0
        dct[t] += 1

ans = -1
for k,v in dct.items():
    if v == 1:
        ans = k
        break
        
print(ans)