N = int(input())
C = {}
for _ in range(N):
    s = list(input())
    for i in range(97,123):
        s1 = s[:]
        s1.append(chr(i))
        s1 = tuple(sorted(s1))
        C[s1] = C.get(s1,0)+1
ans = -1
for s in C:
    if C[s]==1:
        ans = "".join(s)
        break
print(ans)