How do I implement a persisted GraphQL Paginated query that has Int parameters?

I currently have a persisted endpoint that is meant to allow for paging over a list of content fragments: 

/graphql/execute.json/xxx/getSomeContentFragmentPaginated;startRecord=0;numberOfRecords=3

The underlying GraphQL query looks something like this:

query getSomeContentFragmentPaginated($startRecord: Int, $numberOfRecords: Int)
{
     someContentFragmentList(offset: $startRecord, limit: $numberOfRecords) {
          items {
          ...

          }

     }

}

From the GraphQL Editor the query is ok - but when accessing it from the rest endpoint I get a casting error:

Variable 'startRecord' has an invalid value: Expected a value that can be converted to type 'Int' but it was a 'String' 

Any idea how I convert the incoming String value from the url to Int?