Set dynamic id with $index

I've got this code:

My list, which should work like a navigation with main nav points and sub nav points:

list = [
    {
        Id: 1,
        Name: 'Main 1',
        Items: [
            {
                Id: 1,
                Name: 'Sub 1'
            },
            {
                Id: 2,
                Name: 'Sub 2'
            }
        ]
    },
    {
        Id: 2,
        Name: 'Main 2',
        Items: [
            {
                Id: 1,
                Name: 'Sub 1'
            },
            {
                Id: 2,
                Name: 'Sub 2'
            }
        ]
    }
];

And this is my HTML. I loop the main points and it's sub points with ng-repeat and try to set some id's dynamically with the $index.:

<div class="wrapper" ng-repeat="item in list">
    <div id="main_{{$index}}">{{item.Name}}</div>
    <div id="main_{{$parent.$index}}_sub_{{$index}}" ng-repeat="sub in item.Items">
        <div>&mdash; {{sub.Name}}</div>
    </div>
</div>

My result is something like this and it works fine:

Main 1
- Sub 1
- Sub 2
Main 2
- Sub 1
- Sub 2

My goal is now to assign a id to my div with the main and my div with the subs. I add the current index with $index to my id in the main div and after that I try to combine the id for the sub divs with the index from the main (it's parent) and the current index. To get the index of the parent, I used $parent.$index. My expected result are follow id's:

Main 1 -> id = main_0
- Sub 1 -> id = main_0_sub_0
- Sub 2 -> id = main_0_sub_1
Main 2 -> id = main_1
- Sub 1 -> id = main_1_sub_0
- Sub 2 -> id = main_1_sub_1

But this doesn't work at the moment. The id's for the main div is correct, but the id for the sub isn't. I thought with $parent.$index it should work, but there is a problem with the current index of the main. I have no idea how to solve this. I hope someone can help me.

Try the below code snippet, it's working fine here !

var app = angular.module('app',[]);
app.controller('Ctrl',function($scope){
  
$scope.list = [
    {
        Id: 1,
        Name: 'Main 1',
        Items: [
            {
                Id: 1,
                Name: 'Sub 1'
            },
            {
                Id: 2,
                Name: 'Sub 2'
            }
        ]
    },
    {
        Id: 2,
        Name: 'Main 2',
        Items: [
            {
                Id: 1,
                Name: 'Sub 1'
            },
            {
                Id: 2,
                Name: 'Sub 2'
            }
        ]
    }
];

});

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>
<div ng-app="app" ng-controller="Ctrl">
<div class="wrapper" ng-repeat="item in list">
    <div id="main_{{$index}}">{{item.Name}} - main_{{$index}}</div>
    <div id="main_{{$parent.$index}}_sub_{{$index}}" ng-repeat="sub in item.Items">
        <div>&mdash; {{sub.Name}} - main_{{$parent.$index}}_sub_{{$index}}</div>
    </div>
</div>
</div>