How to create an alert when there is a decrease in the metrics

I would like to create an alert of my grafana panel, when this is a decrease of my metrics, it will automatically send alert to my email address.

The metrics is collected daily, duration is nearly 24 hours but not always the same. Could you tell me how can I write the alert rule to achieve this?

currently I wrote it as

with diff() of query(A, 20h, now-20h) is below 0

will this script work?