Switching between nested iframes within a frame

Please help me in resolving the following issue.

Hierarchy of Frameset, frames and iframes in a single web page

Frameset id=”mainframeset”….
frame id=”usersession”…
Frameset id=”lowerframeset”..
Frame id=”Nav”
Frameset =”main pane”…
Frame id=”Work”…
Iframe1 :(each Iframe have attributes frameborder="0",allowtransparency="true",tabindex="0",src="",title="Rich text editor, work_135(only change is this id for each iframe), press ALT 0 for help.",style="width:100%;height:100%"
Iframe2: (each Iframe have attributes frameborder="0", allowtransparency="true",tabindex="0",src="",title="Rich text editor, work_140(only change is this id for each iframe), press ALT 0 for help.",style="width:100%;height:100%"
Iframe3:
Iframe4:

Code I have is :

driver.switchTo().defaultContent();
driver.switchTo().frame("work");
System.out.println(" Switched to Work ");
List Element = driver.findElements(By.xpath("//iframe"));
System.out.println("No of Iframes are :"+ Element.size());
System.out.println(driver.switchTo().frame(Element.get(0)));
driver.findElement(By.xpath("//*[@id='ck_article_title']")).click();
driver.findElement(By.xpath("//*[@id='ck_article_title']/p[1]")).sendKeys("Ifram 1 ");

System.out.println(driver.switchTo().frame(Element.get(1)));
driver.findElement(By.xpath("//*[@id='ck_article_standfirst']")).click();
driver.findElement(By.xpath("//*[@id='ck_article_standfirst']/p[1]")).sendKeys("I frame 2 ");

Error while running the above code:

Exception in thread "main" org.openqa.selenium.StaleElementReferenceException: Element belongs to a different frame than the current one - switch to its containing frame to use it

Problem:

a) The above code enters the text in iframe 1 but gives above error when trying to identify iframe2 within frame "work"

Please help me to identify each iframes sequentially and enter required text in it.