JOLT spec to choose non null field aa result

I'm new to JOLT.

I have below Input :

{
  "F1": "1",
  "F2": "2"
}

I want the output like if F1 is null then it should return F2 value else it should return F1 value..

Expected Output :

{// if F1 is not null
  "result": "1"
}

Input:

{
  "F1": null,
  "F2": "2"
}

Expected Output :

{// as F1 is null
  "result": "2"
}

I tried with isNull but it's not working and not aware how to do

You rather can use notNull function along with Elvis Operator in which the function silently stops if F1 is null, and the argument 2 is returned by using following spec :

[
  {
    "operation": "modify-overwrite-beta",
    "spec": {
      "result": ["=notNull(@(1,F1))", "=(@(1,F2))"]
    }
  },
  {//pick up "result" attribute only
    "operation": "shift",
    "spec": {
      "result": "&"
    }
  }
]