Create plugin for cordova

I try to make a cordova plugin.
I created plugin files with plugman.
This is what I have in MathCalculator.js

var exec = require("cordova/exec");
module.exports.mathCalculator = function (arg0, success, error) {
cordova.exec(success, error, "MathCalculator", "add", [arg0]);
};

In MathCalculator.java I have:

package cordova.plugin.mathcalculator;
import org.apache.cordova.CordovaPlugin;
import org.apache.cordova.CallbackContext;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
/**
* This class echoes a string called from JavaScript.
*/
public class MathCalculator extends CordovaPlugin {
@Override
public boolean execute(String action, JSONArray args, CallbackContext callbackContext) throws JSONException {
if(action.equals("add")) {
this.add(args, callbackContext);
return true;
} else if(action.equals("substract")) {
this.substract(args, callbackContext);
return true;
}
return false;
}
private void add(JSONArray args, CallbackContext callback) {
if(args != null) {
try {
int p1 = Integer.parseInt(args.getJSONObject(0).getString("param1"));
int p2 = Integer.parseInt(args.getJSONObject(0).getString("param2"));
callback.success("" + (p1 + p2));
} catch (Exception ex) {
callback.error("Something went wrong" + ex);
}    
} else {
callback.error("Please do not pass null value");
}
}
private void substract(JSONArray args, CallbackContext callback) {
if(args != null) {
try {
int p1 = Integer.parseInt(args.getJSONObject(0).getString("param1"));
int p2 = Integer.parseInt(args.getJSONObject(0).getString("param2"));
callback.success("" + (p1 - p2));
} catch (Exception ex) {
callback.error("Something went wrong" + ex);
}    
} else {
callback.error("Please do not pass null value");
}
}
}

And in config.xml (I paste here just what is inside <platform name="android"></platform>

<platform name="android">
<config-file parent="/*" target="res/xml/config.xml">
<feature name="MathCalculator">
<param name="android-package" value="cordova.plugin.mathcalculator.MathCalculator" />
</feature>
</config-file>
<config-file parent="/*" target="AndroidManifest.xml" />
<source-file src="src/android/MathCalculator.java" target-dir="src/cordova/plugin/mathcalculator/MathCalculator" />
</platform>

I tried to call this plugin (after add it) in index.js like this:

mathCalculator.add({param1: 1, param2: 3});

But, mathCalculator is not defined.
I exported mathCalculator.

I created the plugin with plugman, then I added in bridge (MathCalculator.js) var

mathCalculator = myMethod: function (options, success, error) {
cordova.exec(success, error, "MathCalculator", "add", [options]);
},
module.exports = mathCalculator;

Then, in cordova I can call this method like this: cordova.plugins.mathCalculator.myMethod();

If I add in plugin xml insite <js-module ...><clobbers target="mathCalculator"></clobbers></js-module>, I can call my plugin directly with mathCalculator name.