set selected value in select list of cshtml using ViewBag

@model StudentAttendance.Models.Semester
<h4>Edit Semester Details </h4>
<div class="row">
<form asp-action="Edit" class="form-horizontal">
<div class="col-md-6">
<div class="form-group">
<label asp-for="SemId" class="control-label col-md-2">Semester Id</label>
<div class="col-md-10">
<input asp-for="SemId" class="form-control" />
<span asp-validation-for="SemId" class="text-danger"></span>
</div>
</div>
<div class="form-group">
<label asp-for="Dos" class="control-label col-md-2">Start Date</label>
<div class="col-md-10">
<input asp-for="Dos" class="form-control" />
<span asp-validation-for="Dos" class="text-danger"></span>
</div>
</div>
<div class="form-group">
<label asp-for="Doe" class="control-label col-md-2">End Date</label>
<div class="col-md-10">
<input asp-for="Doe" class="form-control" />
<span asp-validation-for="Doe" class="text-danger"></span>
</div>
</div>
<div class="form-group">
<label asp-for="DepartmentId" class="control-label">Department</label>
<div class="col-md-6">
<select asp-for="DepartmentId" class="form-control" asp-items="@ViewBag.Dept" > //set option value selected to that of DepartmentId
<option value="">-- Select Department --</option>
</select>
<span asp-validation-for="DepartmentId" class="text-danger" />
</div>
</div>

<div class="form-group">
<input type="submit" value="Save" class="btn btn-default" />
&nbsp;&nbsp;&nbsp;&nbsp; <a asp-action="Index">Back to List</a>
</div>
</div>
</form>
</div>