I want to list articles that belong to several specific categories by selecting a check box.

my domain/category/slugA+slugB

my domain/?category=SlugA%2bSlugB

<form class="cat-search" action="<?php bloginfo('url');?>">
  <input type="checkbox" value="SlugA" id="cat0" class="cats2"/><label for="cat0">Category1</label>
  <input type="checkbox" value="SlugB" id="cat1" class="cats2"/><label for="cat1">Category2</label>
  <input type="checkbox" value="SlugC" id="cat2" class="cats2"/><label for="cat2">Category3</label>
  <input type="checkbox" value="SlugD" id="cat3" class="cats2"/><label for="cat3">Category4</label>
  <input type="checkbox" value="SlugE" id="cat4" class="cats2"/><label for="cat4">Category5</label>
  <input type="hidden" id="cat2" name="category"/>
  <input type="submit" class="cat-search" value="search"/>
</form>

$(function(){
  $('body').on('click', '.cats2', function(){
    varcats=$('.cats2:checked') .map(function(){
      return this.value;
    }).get().join("+");
    $('#cat2') .val(cats);
  });
});