AngularJS Validate Pattern for Latitude

I am trying to validate latitude for the textbox input value entered by a user. The validation fails before a user enters a complete value. i.e. the range of latitude is -90 to +90. As soon as user enters any character for example if user want to enter -10, but as soon as he enters "-" the validation fails, preventing user from entering the complete value.

Here is what I have tried so far. Here is what I would like to achieve. If the validation fails show error, otherwise move onto next item on the form.

<!DOCTYPE html>

<body ng-app="validationApp" ng-controller="myController" class="ng-scope">


<h1>Hello Plunker!</h1>

<div class="form-group">
  <label for="latitude"></label>
  <input autofocus type="text" class=" input" id="latitude" ng-model="latitude" ng-required="true" ng-change="validateLat(latitude)" placeholder='Enter latitude (-90 to +90)' />   
  <!--   <input autofocus type="text" class=" input" id="latitude" ng-model="latitude" ng-required="true" ng-pattern ="myRegex" placeholder='Enter latitude (-90 to +90) ' /> -->


</div>

// Code goes here
var validationApp = angular.module('validationApp', []);

// create angular controller
validationApp.controller('myController', function($scope) {

  /* $scope.myRegex = /^(\\+|-)?(?:90(?:(?:\\.0{1,6})?)|(?:[0-9]|[1-8][0-9])(?:(?:\\.[0-9]{1,6})?))$/;
  if($scope.latitude)
  {
    console.log($scope.latitude)
  }
  else
  {
     window.alert("ERROR:Latitude must be between -90 and 90 degrees inclusive. Please refresh the browser and enter valid Latitude");
  }*/

  $scope.validateLat = function(latitude) {

    var latReg = new RegExp("^(\\+|-)?(?:90(?:(?:\\.0{1,6})?)|(?:[0-9]|[1-8][0-9])(?:(?:\\.[0-9]{1,6})?))$");

    if (latReg.exec(latitude)) {
      console.log(latitude);
    } else {
      window.alert("ERROR:Latitude must be between -90 and 90 degrees inclusive. Please refresh the browser and enter valid Latitude");

    }
  };

});

http://plnkr.co/edit/LNuizqAqL0S6dPdhvnq8[Validation of Latitude in input text box][1]

I would appreciate your help.