public function input_detail_barang()
{
    $id_data = $this->input->post('id_data');
    $no_inv = $this->input->post('no_inv');
    $condition = $this->input->post('condition');

    $data = array
    (
        'id_data' => $id_data,
        'no_inv' => $no_inv,
        'condition' => $condition
    );

    //I need amount from first form, because i will use it. How can i do?

    $amount = $this->session->userdata("Firstformdata");
    echo $amount['amount'];
}