import sys
from collections import deque, Counter
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
inf = 2 ** 63 - 1
mod = 998244353

import random

n = ii()

s = [input() for _ in range(n)]

while True:
    t = ''.join([random.choice('ab') for _ in range(n)])
    if t not in s:
        print(t)
        exit()