[{"result":{"events":[{"amount":[[1623224700000,"33333333"],[1623224760000,"33333333"],[1623224820000,"33333334"]],}],}}]

db.collection.aggregate([{$addFields:{"amount_sum":{$sum:{"$map":{"input":"$result.events","as":"e","in":{$sum:{"$map":{"input":"$$e.amount","as":"a","in":{$toInt:{"$arrayElemAt":["$$a",1]}}}}}}}}}}])