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

val () =
    let
        val l = readLargeInt ()

        val ans = (l - 1) * (l - 1) div 4
    in
        print (LargeInt.toString ans ^ "\n")
    end