One of the powers of MVC is the model binder - which you are completely ignoring here. Create a view model to match the expected content of your view
public class AdditionViewModel
{
public int A { get; set; }
public int B { get; set; }
public int Result { get; set; }
}
Use this as the expected parameter in your action
[HttpPost]
public ActionResult Addition(AdditionViewModel model)
{
model.Result = model.A + model.B;
return View(model);
}
Then finally in your view
@model AdditionViewModel
@using (Html.BeginForm("Addition", "Addition", FormMethod.Post))
{
@Html.EditorFor(x => x.A)
@Html.EditorFor(x => x.B)
@Html.DisplayFor(x => x.Result)
<input type="submit" value="Submit" />
}