In order to ensure the safe transmission of information, the security department adopts the key cryptosystem. The principle of encryption and decryption is as shown in the figure: now the decryption k

In order to ensure the safe transmission of information, the security department adopts the key cryptosystem. The principle of encryption and decryption is as shown in the figure: now the decryption key is x → y = a. The decryption key is: x→ y = ax (a > 0, a≠ 1),

Similarly, the ciphertext "3" was decrypted to get plaintext "8".

Therefore, 8=a3, a=2,

So the decryption key is: x→y=2x,

So when that plaintext input by the input side is "4",

I.e. 4=2x,

The solution is x=2.

So the answer is: 2