葵花寶典教程，一個自學編程平臺

I have this query

 DB::table('product_options')
        ->where('product_id', $id)
        ->where('size', $selected_size)
        ->update(array('stock' => WHAT TO PUT HERE));

In the update part where I've put WHAT TO UPDATE, what should I put here to decrease the number by 1?

DB::table('product_options')
        ->where('product_id', $id)
        ->where('size', $selected_size)
        ->decrement('stock');