import random

def len_t(len = 4):
    c = ""
    for i in range(len):
        r = ["0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
        c = c + r[random.randint(0,36)]

    return c

l = input("请指定验证码长度,不指定则输入回车：")
try:
    l = int(l)
    res = len_t(l)
    print(f"验证码为：{res}")
except ValueError:
    res = len_t()
    print(f"验证码为：{res}")