-- Your code here!

import Control.Monad
import Data.Maybe
import qualified Data.ByteString.Char8 as BS
import Data.List
 
readInt = fst . fromJust . BS.readInt
readIntList = map readInt . BS.words
getInt = readInt <$> BS.getLine
getIntList = readIntList <$> BS.getLine

main=do
    [a,b]<-getIntList
    print $ ((a+b)*(a+b-1))`div`2-a*b
    