import random
import collections

N = int(input())
NG = [input() for _ in range(N)]
WORD = collections.deque()
for _ in range(N):
    if random.random()>0.5:
        WORD.append('a')
    else:
        WORD.append('b')
while True:
    for ng in NG:
        if list(ng)==list(WORD):#滅茶苦茶重いはず
            break
    else:
        print(''.join(WORD))
        exit()
    WORD.popleft()
    if random.random()>0.5:
        WORD.append('a')
    else:
        WORD.append('b')