fun	  unfold 0 = []
	| unfold n = let val ((q:int), (r:int)) = (n div 2, n mod 2)
		     in (unfold q) @ [if r = 1 then true else false]
		     end

fun crunch (p, (f, g)) = if p then (f*(f+2*g), f*f+g*g) else (f*f+g*g, g*(2*f-g))

fun myfib n = #2 (foldl crunch (1, 0) (unfold n))

val n = valOf(Int.fromString(hd(CommandLine.arguments())))
  handle Option =>
    (print ("usage: tailfib n\n");
    OS.Process.exit(OS.Process.failure))

val _ = print (IntInf.toString (myfib n) ^ "\n")