from itertools import combinations
from cgitb import reset
from collections import defaultdict
from functools import lru_cache
from sys import flags, stdin
import math
import re
import queue
from tokenize import String
import typing
import itertools
import bisect
import statistics
# import numpy as np
# from numpy.core.function_base import _needs_add_docstring
# from numpy.core.numeric import outer
input = stdin.readline
MOD = 1000000007
INF = 122337203685477580


def solve():
    n = int(input())
    # set
    st = set()
    for _ in range(n):
        # read string remove line break
        s = input().rstrip()
        st.add((s))

    # print(st)

    nn = min(10, n)
    for i in range(2**nn):
        s = ""
        for j in range(0, nn):
            if (i >> j) & 1:
                s += "a"
            else:
                s += "b"
        while len(s) < n:
            s += "a"
        if s not in st:
            print(s)
            return

    return


if __name__ == '__main__':
    solve()