
import qualified Data.ByteString.Char8 as C
import Data.List
import Control.Monad

import qualified Data.IntMap as M
import qualified Data.Sequence as S

import Data.Foldable(toList)
import Data.Maybe(fromMaybe)


r :: IO [Int]
r  = unfoldr (C.readInt . C.dropWhile (==' ')) <$> C.getLine

rp :: IO (Int,Int)
rp  = toTup . unfoldr (C.readInt . C.dropWhile (==' ')) <$> C.getLine
    where toTup [a,b] = (a,b)


main :: IO ()
main = do
    n <- readLn :: IO Int
    print $ div (n*(n-1)) 2

