In the task said "The first line consists of 2N characters and represents the encoded message N bytes long".
But I understood that if we encode the message length of n, we will receive message the same length (n). Whats wrong?

Each byte can be presented as 2 characters in a hexadecimal form

0 - 00
1 - 01
..
16 - 10
..
255 - FF

so n bytes = 2*n characters

Thank you! Now I'll try to solve it!