Capture data with Selenium, add and insert into another field

I have a code where it captures data from one field, using Selenium WebDriver and inserts into another field.

I would like to add (captured field + 1000) and then insert into another field.

Where will I enter + 1000?

WebElement ultimokm = driver.findElement(By.id("customfield_23430"));// captura dados no campo
String getultimokmtext = ultimokm.getAttribute("value");
WebElement kmatual = driver.findElement(By.id("customfield_17202"));
kmatual.clear();
kmatual.sendKeys(getultimokmtext);