Sum of frontmatter metadata from Sub-Notes / twig

[e-frank]

i am looking for a way to present the sum of hours stored in sub-notes frontmatter, like:

---
title: Important Task
hours:
  - 4
---

├── Summary
│   ├── Task #1
│   ├── Task #2
│   ├── Task #n

what i am looking for:

=sum(frontmatter.hours)

i tried using pandoc templating, but i seems it is overwriting frontmatter variables instead of merging.

or even better, something like twig:

{% set totalHours = 0 %}

{% for entry in entries %}
    {% set totalHours = totalHours + entry.hours %}
{% endfor %}

Total Hours: {{ totalHours }}

how can this be achieved?
thanks in advance for your help!