N = int(input())
S = []
for i in range(N):
    s = input()
    t = list(s)
    t.sort()
    s = ''.join(t)
    S.append(s)
S.sort()
T = []
for i in range(N):
    if S.count(S[i]) == 1:
        T.append(S[i])
# print(T)
U = []
for t in T:
    for i in range(26):
        c = chr(ord("a")+i)
        u = list(t)
        u.append(c)
        u.sort()
        u = ''.join(u)
        U.append(u)
# print(U)
for u in U:
    if u not in S:
        print(u)
        exit()
print(-1)