Declarative Lookup Rollup Summaries (DLRS) is a community built and maintained Salesforce application that allows you to create cross object roll-ups declaratively - no code! For install instructions and documentation, visit our website https://sfdo-community-sprints.github.io/DLRS-Documentation/
Why do you think that https://github.com/SFDO-Community/Summit-Events-App is a good alternative to declarative-lookup-rollup-summaries