Usage of special character "@" in JOLT template

Please help me in writing the jolt spec to get the expected output with special character "@" as shown below. When I place "@FirstName":"Names.FirstName" in the spec it is not working. Please help in resolving this.

JSON Input:

{
    "FirstName": "First"
}

Jolt Spec:

 [
      {
        "operation": "shift",
        "spec": {
          "FirstName": "Names.FirstName"
        }
      }
    ]

Expected Output:

{
  "Names" : {
    "@FirstName" : "First"
  }
}