fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)

fun int_string n =
    if 0 <= n
    then Int.toString n
    else "-" ^ Int.toString (abs n)

val () =
    let
        val n = readInt ()
    in
        print (int_string (n * (n + 1) div 2))
    end