amazon404.user.js

// ==UserScript==
// @name         Amazon 404 Fuck
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://www.amazon.co.jp/*/dp/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=amazon.co.jp
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    const path = location.pathname.split("/").pop()
    location.pathname = `/dp/${path}`;

    // Your code here...
})();