Problem with CI4 session

please help.

i have a libary code like bellow

libarary

<?php namespace App\Libraries\Mylibrary;

class Mylibrary{

function hitung(){
$angka = 2;
return $angka;
}
}

controller :

<?php namespace App\Controllers;

use App\Libraries\Mylibrary;

class Home extends BaseController
{

function sesi(){

$this->session->set('abc','item');
if ($this->session->get('abc')!='item') {
echo "string";
}else{
echo "strong";
}

echo hitung();
}
}

the error

Call to undefined function App\Controllers\hitung()

thanks.