Given this pipeline on a collection of orders, what is the value of the 'total' field in the output group?
db.orders.insertMany([
{ amount: 1 },
{ amount: 2 },
{ amount: '10' },
{ other: 5 }
]);
db.orders.aggregate([
{ $group: { _id: null, total: { $sum: '$amount' } } }
]);