protected function getApiEndpoint()
    {
        $servers = array();
        if ($this->options['sandbox']) {
            $servers[] = self::SANDBOX_SERVER;
        } else {
            $servers = array(self::MAIN_SERVER, self::BACKUP_SERVER);
        }

        foreach ($servers as $server) {
            $doc = new \DOMDocument();
            $doc->loadHTMLFile('https://'. $server . "/load.html");

            $element = $doc->getElementById('server_status');
            if ($element && 'OK' == $element->textContent) {
                return $server;
            }
        }

        throw new RuntimeException('No server available.');
    }