Converting JSON to specific string format

I get the following json data from server.

{"visits":[{"City":6,"Count":5},{"City":16,"Count":1},{"City":23,"Count":1},{"City":34,"Count":1}]}

and i need to convert it to following format:

{"1":"82700","2":"26480","3":"31530","4":"22820","5":"15550","6":"205790"}

I have the following code but not working out:

       var cities = "{";

        for (var key in data.visits) {
            var val = data.visits[key];
            //Now you have your key and value which you 
            //can add to a collection that your plugin uses
            var obj = {};
            obj[val.City] = '' + val.Count;

            var code = '' + val.City;
            var count = '' + val.Count;

            cities += code + ':' + count + ',';
        }

        cities += "}";

I need the integers in string representation and need to get rid of the final , .

How can i fix this?