Your login page should be in such format :
<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="Login.aspx.cs" Inherits="Login" Title="Login" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
Paste Here all the style code of master page
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID=ContentPlaceHolder1; Runat="Server">
code of Login Page
</asp:Content>