How to compare parent field value with aggregated values from nested objects in Elasticsearch?

I have such documents in my index:

  {
    "Pieces": 5,
    "parts": [
      {
        "Quantity": 1,
        "PartID": 1,
      },
      {
        "Quantity": 1,
        "PartID": 2
      },
      {
        "Quantity": 1,
        "PartID": 3
      },
      {
        "Quantity": 1,
        "PartID": 4
      }
    ]
  }

I would like to prepare query which lists all docs where Pieces value does not equal to the sum of Quantity in all parts. I have something like that:

GET /_search
{
  "size": 0,
  "aggs": {
    "group_by_id": {
      "terms": {
        "field": "_id",
        "size": 1000000000
      },
      "aggs": {
        "sum_of_parts": {
          "nested": {
            "path": "parts"
          },
          "aggs": {
            "sum_of_quantity": {
              "sum": {
                "field": "parts.Quantity"
              }
            }
          }
        },
        "sum_of_parts_vs_pieces": {
          "bucket_selector": {
            "buckets_path": {
              "sumOfPart": "sum_of_parts>sum_of_quantity"
            },
            "script": "params.sumOfPart != HOW_TO_PUT_Pieces_VALUE_HERE???"
          }
        }
      }
    }
  }
}

which almost works (counts the sum of Quantity for all parts) ... but... I have no idea how to pass Pieces value to the bucket_selector.

Do you have any idea how to do this? Or maybe you have an idea of how to accomplish this task in another way?

You can do it like this, by adding the average of the Pieces field (going back up one level using reverse_nested):

GET pieces/_search
{
  "size": 0,
  "aggs": {
    "group_by_id": {
      "terms": {
        "field": "_id",
        "size": 1000000000
      },
      "aggs": {
        "sum_of_parts": {
          "nested": {
            "path": "parts"
          },
          "aggs": {
            "avg_of_pieces": {
              "reverse_nested": {},
              "aggs": {
                "avg_of_pieces": {
                  "avg": {
                    "field": "Pieces"
                  }
                }
              }
            },
            "sum_of_quantity": {
              "sum": {
                "field": "parts.Quantity"
              }
            }
          }
        },
        "sum_of_parts_vs_pieces": {
          "bucket_selector": {
            "buckets_path": {
              "sumOfPart": "sum_of_parts>sum_of_quantity",
              "nbPieces": "sum_of_parts>avg_of_pieces>avg_of_pieces"
            },
            "script": "params.sumOfPart != params.nbPieces"
          }
        }
      }
    }
  }
}