Get WordPress Menu top-most Parent ID

<?php
//functions.php file function 

function get_menu_parent_ID($menu_name){
    if(!isset($menu_name)){
      return "No menu name provided in arguments";
    }
    $menu_slug = $menu_name;
    $locations = get_nav_menu_locations();
    $menu_id   = $locations[$menu_slug];
    $post_id        = get_the_ID();
    $menu_items     = wp_get_nav_menu_items($menu_id);
    $parent_item_id = wp_filter_object_list($menu_items,array('object_id'=>$post_id),'and','menu_item_parent');
    $parent_item_id = array_shift( $parent_item_id );
    function checkForParent($parent_item_id,$menu_items){
      $parent_post_id = wp_filter_object_list( $menu_items, array( 'ID' => $parent_item_id ), 'and', 'object_id' );
      $parent_item_id = wp_filter_object_list($menu_items,array('ID'=>$parent_item_id),'and','menu_item_parent');
      $parent_item_id = array_shift( $parent_item_id );
      if($parent_item_id=="0"){
        $parent_post_id = array_shift($parent_post_id);
        return $parent_post_id;
      }else{
        return checkForParent($parent_item_id,$menu_items);
      }
    }
    if(!empty($parent_item_id)){
      return checkForParent($parent_item_id,$menu_items);
    }else{
      return $post_id;
    }
  }
  
  // Fire the function
  
  $parentID = get_menu_parent_ID('main_nav');
  
  ?>