Dynamic Web table with pagination

We have a dynamic web table with pagination which expand or collapse depend on the customer requests,How to i get total number of row for this table.

int i=1;
int totalRows=0;
String xpath_start="//div[@id='ext-gen255']/div[";
String xpath_end="]/table/tbody/tr/td[1]/div";

while((driver.findElement(By.xpath(xpath_start+i+xpath_end )) != null))
{
i++;
totalRows++;
}
System.out.println("Total rows -- " +totalRows);

when i run this code it is taking up to 25 rows as per pagination ,26th row throwing error "Unable to locate element: {"method":"xpath","selector":"//div[@id='ext-gen255']/div[26]/table/tbody/tr/td[1]/div"}[/color]