How implementing a directive to validate another field (custom form in angularjs)

I have a validation custom form, I use validate-message-character="{{compose.limitCharacter - compose.message.length}}" in a select and textarea like this

            <form class="form-horizontal" role="form" name="composeForm" >

            <select ng-change="limitComposeCharacter()" ng-selected="compose.profile" 
             ng-model="compose.profile" ng-options="userId in profiles" name="profile" 
             validate-message-character="{{compose.limitCharacter - compose.message.length}}" required>
            </select>

               number Character: {{compose.limitCharacter - compose.message.length}}

            <textarea class="form-control" ng-model="compose.message" name="message" 
    validate-message-character="{{compose.limitCharacter - compose.message.length}}" 
required></textarea>
              Form validity: {{composeForm.$valid}}

I have something like this:

• 1° Select User has compose.limitCharacter = 100
• 2° Select User has compose.limitCharacter = 200 etc etc.

This is my directive to check number Character is > 0

angular.module('App')
  .directive('validateMessageCharacter', function () {
    return {
      require: 'ngModel',
      link: function postLink(scope, element, attrs, c) {
        scope.$watch(attrs.ngModel, function() {
            console.log(attrs.validateMessageCharacter);
            if(attrs.validateMessageCharacter < 0)
            {
                c.$setValidity('maxCharacter', false);
                c.$invalid = true;
            }else{
                c.$setValidity('maxCharacter', true);
                c.$invalid = false;
            }

        });
      }
    };
  });

It doesn't work proply when change select without change the textarea some advice?