Got Http response code 400. Problem with Amount. Error using PAYPAL API

is the first time I try to make a page implementing paypal and it is giving me problems, I use Laravel too.

This is the function that gives me the error

private $_api_context;

    public function __construct()
    {

        // setup PayPal api context
        $paypal_conf = \Config::get('paypal');
        $this->_api_context = new ApiContext(new OAuthTokenCredential($paypal_conf['client_id'], $paypal_conf['secret']));

        $this->_api_context->setConfig($paypal_conf['settings']);
    }

    public function postPayment()
    {
          $payer = new Payer();
         $payer->setPaymentMethod('paypal');
         $elementos = array();
         $subtotal = 0;
         $carrito = \Session::get('carrito');
         $monedaActual = 'EUR';

         foreach ($carrito as $producto):

            $Item = new item();
            $Item->setName($producto->nombreproducto)
                        ->setCurrency($monedaActual)
                        ->setDescription($producto->descbreve)
                        ->setQuantity($producto->cantidad)
                        ->setPrice($producto->precio);
                 $elementos[] = $Item;
                  $subtotal += $producto->cantidad + $producto->precio;     
         endforeach;



        $listaElementos = new ItemList();
        $listaElementos->setItems($elementos);

        $detalles = new Details();
        $detalles->setSubtotal($subtotal)
                     ->setShipping(100);

        $total = $subtotal + 100;

        $amount = new Amount();
        $amount->setCurrency($monedaActual)
                   ->setTotal($total)
                   ->setDetails($detalles);

        $transaccion = new Transaction();
        $transaccion->setAmount($amount)
                    ->setItemList($listaElementos)
                    ->setDescription('Pedido de prueba de la aplicacion Laravel ');

      $urlsRedirecionadas  = new RedirectUrls();
      $urlsRedirecionadas->setReturnUrl(\URL::to('payment/status'))
                          ->setCancelUrl(\URL::to('payment/status'));

      $Pago = new Payment();
      $Pago->setIntent('sale')
            ->setPayer($payer)
            ->setRedirectUrls( $urlsRedirecionadas)
            ->setTransactions(array($transaccion));


      try
       {
          $Pago->create($this->_api_context);
        } catch (\PayPal\Exception\PayPalConnectionException $ex)
       {
          if (\Config::get('app.debug')) :
               echo 'Excepcion '.$ex->getMessage().PHP_EOL;
                $err_data = json_decode($ex->getData(), true);



                exit;
           else :
              die('Algo ha fallado, compruebe si estan correctos todos los pasos');
           endif;


  return \Redirect::away('/carrito')
                        ->with('mensaje','Hay un error desconocido');




    }

The error of the paypal log is this

[23-08-2016 10:26:21] PayPal\Core\PayPalHttpConnection : ERROR: Got Http response code 400 when accessing https://api.sandbox.paypal.com/v1/payments/payment. {"name":"VALIDATION_ERROR","details":[{"field":"transactions[0]","issue":"Item amount must add up to specified amount subtotal (or total if amount details not specified)"}],"message":"Invalid request - see details","information_link":"https://developer.paypal.com/docs/api/#VALIDATION_ERROR","debug_id":"c486cbbedfec1"}

and the debug of the variable $ Payment in json is this

"{"intent":"sale","payer":{"payment_method":"paypal"},"redirect_urls":{"return_url":"http://localhost/HTDOCS_PHP/laravel_tienda_overfiso/public/payment/status","cancel_url":"http://localhost/HTDOCS_PHP/laravel_tienda_overfiso/public/payment/status"},"transactions":[{"amount":{"currency":"EUR","total":"601.28","details":{"subtotal":"501.28","shipping":"100"}},"item_list":{"items":[{"name":"perico_2","currency":"EUR","description":"mensaje descripcion 2","quantity":1,"price":"500.28"}]},"description":"Pedido de prueba de la aplicacion Laravel "}]}"