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


fun quicksort [] = []
  | quicksort (h::tl) =
    let 
        val (s, b) =
            List.foldl
                (fn (x, (small, big)) =>
                    if x <= h then (x::small, big)
                    else (small, x::big))
                ([], [])
                tl
    in
        (quicksort s) @ [h] @ (quicksort b)
    end

fun isContinuous [] = true
  | isContinuous [_] = true
  | isContinuous (h1 :: h2 :: tl) =
    if h1 + 1 = h2 then isContinuous (h2 :: tl)
    else false


val () =
    let
        val a = readInt ()
        val b = readInt ()
        val c = readInt ()
        val d = readInt ()

        val sorted = quicksort [a, b, c, d]

        val ans = if isContinuous sorted then "Yes"
                  else "No"
    in
        print (ans ^ "\n")
    end