How to deserialize into a enum variant based on a key name?

I have JSON which takes two forms:

"Example:" { "field": 42, "A": 76 }
"Example:" { "field": 42, "B": 110 }

I want to deserialize it into a struct like this:

struct Example {
    field: i32,
    an_enum: AnEnum,
}

where

enum AnEnum {
    A(i32),
    B(i32),
}

I don't know how to do it without writing a custom deserializer for Example.

This works:

"Example:" { "field": 42, "an_enum": {"A": 76} }

or, in YAML:

Example:
    field: 42
    an_enum:
        A: 76

The an_enum is superfluous and annoying to write. How can I deserialize the first form into the struct? Or, alternatively, how can I declare a struct that will successfully deserialize the syntax I want?