Lodash in Angular JS - getting TypeError: Object function lodash(value)

i've created an application in angular for performing a simple calculator. The application works fine (demo) but here in the controller within the calculate method i've wrote some arithmetic calculation in javascript. So to make my angular code clean i created another file named common.js where i placed that arithmetic calculations. I'm using lodash.js. but when i tried to call the method _.calculateResult($scope.firstNumber, $scope.secondNumber, $scope.selectedOperator ); i'm getting the following exception.

TypeError: Object function lodash(value) {
      // don't wrap if already wrapped, even if wrapped by a different `lodash` constructor
      return (value && typeof value == 'object' && !isArray(value) && hasOwnProperty.call(value, '__wrapped__'))
       ? ...<omitted>... } has no method 'calculateResult'
    at Object.$scope.calculate (http://localhost:8080/RestSample/app/scripts/controllers.js:18:19)

can anyone please tell me some solution for this.

common.js

(function() {

var calculateResult = function(no1, no2, opp) {
var A = parseInt(no1);
var B = parseInt(no2);
var C = 0;

switch (opp) {
      case '+':
      C = A + B;
      break;
      case '-':
      C = A - B;
      break;
      case '*':
      C = A * B;
      break;
      case '/':
      C = A / B;
      break;                
   }

   return C;
}

})();

controllers.js

    var app = angular.module('app', []);

    app.controller("appController", function($scope){
        $scope.operators = ['+', '-', '*', '/'];
        $scope.selectedOperator = $scope.operators[0];

        $scope.calculate = function() {
$scope.result = _.calculateResult($scope.firstNumber, $scope.secondNumber, $scope.selectedOperator );

        };
    });

index.html

    <div ng-app="app">
        <div ng-controller="appController">

     <div class="offset4 span6 well">
     <label>Enter a value :</label>
     <input ng-model="firstNumber" type="text"> <br/><br>


     <label>Another Value:</label>
     <input ng-model="secondNumber" type="text"> <br/><br>

     <label>Operator:</label>
     <select ng-model="selectedOperator" 
     ng-options="operator for operator in operators"></select> 
     <br><br>

     <button ng-click="calculate()">Calculate</button> <br><br>

     Result: {{result}}
     </div>
     </div>
    </div>