@using System.Web.Helpers
@{
    Layout = null;
    string result = "Choose 1, 2 or 3";
    if (IsPost) {
        try { AntiForgery.Validate(); }
        catch (System.Web.Mvc.HttpAntiForgeryException) { Response.StatusCode = 400; Response.Write("Token refused"); Response.End(); }
        var values = Request.Form.GetValues("quantity");
        if (values == null || values.Length != 1 || (values[0] != "1" && values[0] != "2" && values[0] != "3")) {
            Response.StatusCode = 400; result = "Invalid quantity";
        } else { result = "Requested: " + values[0]; }
    }
}
<form method="post">@AntiForgery.GetHtml()<label>Quantity <input name="quantity" value="1" /></label><button>Check</button></form>
<p id="result">@result</p>