import sequtils,strutils,math
var
    N = stdin.readline.parseInt
    A = stdin.readline.split.map(parseInt)
    ans : float = 0
    P : array[101, array[101, array[101, float]]]
    c : array[3, int]
P[0][0][1] = N.float

proc solve(a, b, c : int) : float =
    var k = N - a - b - c
    if k == N:
        return 0
    if P[a][b][c] > 0:
        return P[a][b][c]
    if a > 0:
        result += solve(a - 1, b + 1, c) * (a / (a + b + c))
    if b > 0:
        result += solve(a, b - 1, c + 1) * (b / (a + b + c))
    if c > 0:
        result += solve(a, b, c - 1) * (c / (a + b + c))
    result += N / (a + b + c)
    P[a][b][c] = result


for a in A:
    if a > 2:
        continue
    c[a] += 1
ans = solve(c[0], c[1], c[2])

echo ans