java.lang.String cannot be converted to JSONObject?

I´m new to programming and android and some problems with JsonObject/JsonArray, I know Im close but missing something .

After trying some solutions and tuts, I have no errors ! , even If I try others api I get it all right , but in this case the app is closing .

OK, I use this api , this is all the response

{ "target": "MXN", "success": true, "rate": 13.1035, "source": "USD", "amount": 13.1, "message": "" }

I understand my error is in the “cannot be converted to JSONObject”, but I don´t know how to fix it ? Any Ideas ?

my logcat

org.json.JSONException: Value MXN at target of type java.lang.String cannot be converted to JSONObject
at org.json.JSON.typeMismatch(JSON.java:100)
at org.json.JSONObject.getJSONObject(JSONObject.java:578)
at com.example.androidjsonparser.MainActivity$ProgressTask.doInBackground(MainActivity.java:106)
at com.eaxmple.androidjsonparser.MainActivity$ProgressTask.doInBackground(MainActivity.java:1)

MainActivity

package com.example.androidjsonparser;

import java.util.ArrayList;...

public class MainActivity extends ListActivity {
private Context context;

private static String url = "http://currency-api.appspot.com/api/USD/MXN.json?key=24c050085728994c96582a3000b9673f55370909";

private static final String TAG_TARGET = "target";
private static final String TAG_RATE = "rate";
private static final String TAG_SOURCE = "source";
private static final String TAG_AMOUNT = "amount";
private static final String TAG_MESSAGE = "";

ArrayList<HashMap<String, String>> jsonlist = new ArrayList<HashMap<String, String>>();

ListView lv ;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    new ProgressTask(MainActivity.this).execute();
}

private class ProgressTask extends AsyncTask<String, Void, Boolean> {
    private ProgressDialog dialog;

    private ListActivity activity;

    // private List<Message> messages;
    public ProgressTask(ListActivity activity) {
        this.activity = activity;
        context = activity;
        dialog = new ProgressDialog(context);
    }

    /** progress dialog to show user that the backup is processing. */

    /** application context. */
    private Context context;

    protected void onPreExecute() {
        this.dialog.setMessage("Progress start");
        this.dialog.show();
    }

    @Override
    protected void onPostExecute(final Boolean success) {
        if (dialog.isShowing()) {
            dialog.dismiss();
        }
        ListAdapter adapter = new SimpleAdapter(context, jsonlist,
                R.layout.list_item, new String[] { TAG_TARGET, TAG_RATE,
                TAG_SOURCE, TAG_AMOUNT }, new int[] {
                R.id.target, R.id.rate, R.id.source,
                R.id.amount });

        setListAdapter(adapter);

        // selecting single ListView item
         lv = getListView();

    }

    protected Boolean doInBackground(final String... args) {

        JSONParser jParser = new JSONParser();

        // getting JSON string from URL
        JSONObject json = jParser.getJSONFromUrl(url);

        for (int i = 0; i < json.length(); i++) {
            try {                   
                JSONObject c = json.getJSONObject(TAG_TARGET);

                String target = c.getString(TAG_TARGET);
                String rate = c.getString(TAG_RATE);
                String source = c.getString(TAG_SOURCE);
                String amount = c.getString(TAG_AMOUNT);                    
                String message = c.getString(TAG_MESSAGE);

                HashMap<String, String> map = new HashMap<String, String>();

                // adding each child node to HashMap key => value
                map.put(TAG_TARGET, target);
                map.put(TAG_RATE, rate);
                map.put(TAG_SOURCE, source);
                map.put(TAG_AMOUNT, amount);

                map.put(TAG_MESSAGE, message);

                jsonlist.add(map);
            } catch (JSONException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

        return null;

    }

}

}

And my JsonParser

package com.example.androidjsonparser;

import java.io.BufferedReader;...

public class JSONParser {  

static InputStream is = null;
static JSONObject jarray = null;
static String json = "";

// constructor
public JSONParser() {  

}

public JSONObject getJSONFromUrl(String url) {

       StringBuilder builder = new StringBuilder();
        HttpClient client = new DefaultHttpClient();
        HttpGet httpGet = new HttpGet(url);
        try {
          HttpResponse response = client.execute(httpGet);
          StatusLine statusLine = response.getStatusLine();
          int statusCode = statusLine.getStatusCode();
          if (statusCode == 200) {
            HttpEntity entity = response.getEntity();
            InputStream content = entity.getContent();
            BufferedReader reader = new BufferedReader(new InputStreamReader(content));
            String line;
            while ((line = reader.readLine()) != null) {
              builder.append(line);
            }
          } else {
            Log.e("==>", "Failed to download file");
          }
        } catch (ClientProtocolException e) {
          e.printStackTrace();
        } catch (IOException e) {
          e.printStackTrace();
        }

    // try parse the string to a JSON object
    try {
        jarray = new JSONObject( builder.toString());
    } catch (JSONException e) {
        Log.e("JSON Parser", "Error parsing data " + e.toString());
    }

    // return JSON String
    return jarray;

}
 }