public function getOfferSN() 
    {
        $start = 5000;
        $step = 5000;
        $max = Offer::max('sn');
        $max = count($max) && $max > $start ? $max : $start;
        $sn = $max+1;
        $sn_str = strval($sn);
        if(str_contains($sn_str, '4')) {
            $sn_str = str_replace('4', '5', $sn_str);
        }
        $sn = intval($sn_str);

        $in_goods = Goods::where('sn', $sn)->first();
        $max_goods = Goods::max('sn');

        if(count($in_goods)) {
            $sn = $step * ceil($max_goods / $step)+ $step;
        }
        return $sn;
    }