8. Jon created a unique encryption method, where each digit of the plaintext number is transformed into a digit of the ciphertext number by "multiplying by 3, adding 1, and taking the unit digit," for example, the plaintext "27" is encrypted to the ciphertext "72". If a four-digit plaintext number, after two rounds of encryption, becomes the ciphertext "2019", then this four-digit plaintext number is $\qquad$