fun readLargeInt () =
    valOf (TextIO.scanStream (LargeInt.scan StringCvt.DEC) TextIO.stdIn)


val () =
    let
        val n = readLargeInt ()

        fun findAns x acc =
            if n < x then acc - 1
            else findAns (x + 1) (LargeInt.max ((x + 1) * (n - x + 1), acc))
                        
        val ans = findAns 0 0
    in    
        print (LargeInt.toString ans ^ "\n")
    end