Facing an error in MVC DropDownList

Question

I'm new here on STACKOVERFLOW...

I'm just facing an error during adding a Student in the selected DDL item..

Model 1: Department

public class Department
{
    [Key]
    public int deptid { get; set; }
    public string deptname { get; set; }

    public IEnumerable<Student> Student { get; set; }
}

Model 2: Student

public class Student
{
    [Key]
    public int stdid { get; set; }
    public string stdname { get; set; }
    public string stdaddress { get; set; }

    [ForeignKey("Department")]
    public int deptid { get; set; }

    public Department Department { get; set; }
}

DepartmentsController:

public class DepartmentsController : Controller
{
    // GET: Departments
    public ActionResult AddDepartment()
    {
        return View();
    }
    [HttpPost]
    public ActionResult AddDepartment(Department model)
    {
        if (ModelState.IsValid)
        {
            using (DbAccess db=new DbAccess())
            {
                db.Departments.Add(model);
                db.SaveChanges();
            }
        }
        return View(model);
    }
}

StudentsController:

public class StudentsController : Controller
{
    // GET: Students
    public ActionResult AddStudent()
    {
        DbAccess db = new DbAccess();
            SelectList datalist = new SelectList(db.Departments.ToList(), "deptid", "deptname");
            ViewBag.deptid = datalist;
            return View();


    }
    [HttpPost]
    public ActionResult AddStudent(Student model)
    {
        if (ModelState.IsValid)
        {
            using (DbAccess db = new DbAccess())
            {
                db.Students.Add(model);
                db.SaveChanges();
            }
        }
        return View(model);
    }
}

Students View:

@model MVC_Practice2.Models.Student

@{
ViewBag.Title = "AddStudent";
Layout = "~/Views/Shared/_Layout.cshtml";
SelectList datalist = ViewBag.deptid;
}
.
.
.

<div class="form-group">
        @Html.LabelFor(model => model.deptid, "deptid", htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">              
            @Html.DropDownListFor(m=>m.deptid,datalist)
            @Html.ValidationMessageFor(model => model.deptid, "", new { @class = "text-danger" })
        </div>
    </div>

So when I add a student in the selected department from DDL I faced the below error:

The ViewData item that has the key 'deptid' is of type 'System.Int32' but must be of type 'IEnumerable<SelectListItem>'.

And the below line highlighted:

@Html.DropDownListFor(m=>m.deptid, datalist)

Error Screenshot