Displaying count of documents in SingleStat panel

I need some basic help to with elasticsearch.
I added a SingleStat panel where I want to display the number (count) of issues with status open

How should I build the Lucene query?

(expected result: 1)

Those are the documents in my index (from elasticsearch):

"hits" : {
    "total" : 3,
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "issues",
        "_type" : "issue",
        "_id" : "i1",
        "_score" : 1.0,
        "_source" : {
          "number" : "9874",
          "post_date" : "2017-08-12T02:35:01",
          "status" : "closed"
        }
      },
      {
        "_index" : "issues",
        "_type" : "issue",
        "_id" : "i2",
        "_score" : 1.0,
        "_source" : {
          "number" : "1234",
          "post_date" : "2008-10-25T10:10:22",
          "status" : "closed"
        }
      },
      {
        "_index" : "issues",
        "_type" : "issue",
        "_id" : "i3",
        "_score" : 1.0,
        "_source" : {
          "number" : "39001",
          "post_date" : "2009-11-15T14:12:12",
          "status" : "open"
        }
      }
    ]   }

Filter on status open, in options tab pick value current