Transform string to integer in NiFi

How can I transform a string to an integer in NiFi? I tried JoltTransform, but the string is transformed to a negative (??) integer. In the example, below, the timestamp_start original string value 1677720088000 is transformed to -1612124736. What am I missing in my Jolt specification? Any help is appreciated. Is there an alternative to the JoltTransform - another processor?

Looks like this is a known issue: https://github.com/bazaarvoice/jolt/issues/749

Original JSON

Transformed JSON

Jolt Specification

[
  {
    "operation": "modify-overwrite-beta",
    "spec": {
      "timestamp_start": "=toInteger"
    }
  },
  {
    "operation": "default",
    "spec": {}
  }
]