# NGワード回避


def main():

    N = int(input())
    NG = [input().strip() for _ in range(N)]


    for bit in range(2**N):
        words = ""

        for i in range(N):
            if bit >> i & 1:
                words += "b"
            else:
                words += "a"

        if words not in NG:
            print(words)
            exit()





if __name__ == "__main__":
    main()