Web api is sending JSON object in wrong format

I'am working with DevExtreme. I'am trying receive JSON from web api server using ajax.

function getJSONfunction()
{
$.ajax({
    url: 'http://localhost:999/api/CheckNotification?machinename=LENOVO-PC',
    type: "Get",
    dataType: 'json',
    success: function (data) {
        alert('ok');
    }
})

I am expecting received from web api json object like this:

{"Result":"true"}

But problem is, web api is sending object like this:

"{\"Result\":\"true\"}"

And I can't see alert from getJSONfunction().

In web api "Get" function look like this:

public string Get(string machineName)
    {
        int NotificationsNumber = NotificationsFunctions.CheckForNotifications(machineName);
        NotificationsResult result = new NotificationsResult();
        if(NotificationsNumber > 0)
        {
            result.Result = "true";
        }else
        {
            result.Result = "false";
        }
        JavaScriptSerializer js = new JavaScriptSerializer();
        string json = js.Serialize(result);
        return json;
    }

where "NotificationsResult" is a class

public class NotificationsResult
{
    public string Result { get; set; }
}

And my question is how can I received JSON object from api in correct format?