import future, strutils, algorithm


proc nextString: string =
    result = ""
    while not endOfFile stdin:
        let c = readChar stdin
        if c == ' ' or c == "\n"[0]: break
        elif c != '\r': add result, c


proc nextInt: int =
    parseInt nextString()


let
    n = nextInt()

var
    A, B = lc[nextInt() | (i <- 0 ..< n), int].sorted(cmp)
    win = 0
    cnt = 0

proc fight: void =
    var miniWin = 0
    inc cnt
    for i in 0 ..< n:
        if A[i] > B[i]:
            inc miniWin
        else:
            dec miniWin
    if miniWin > 0:
        inc win


proc main: void =
    fight()
    while B.nextPermutation:
        fight()
    while A.nextPermutation:
        sort(B, cmp)
        fight()
        while B.nextPermutation:
            fight()

    echo((win / cnt).formatFloat(ffDecimal, 2))

when isMainModule:
    main()