from collections import Counter

N = int(input())
cnt = Counter()

for _ in range(N):
    T = input()
    for i in range(ord("a"),ord("z")+1):
        S = list(T)
        S.append(chr(i))
        S.sort()
        cnt["".join(S)] += 1

for key, c in cnt.items():
    if c == 1:
        print(key)
        exit()

print(-1)