import sys
from functools import lru_cache

def main():
    n = int(sys.stdin.readline())
    a = list(map(int, sys.stdin.readline().split()))
    
    m3_initial = 0
    m2_initial = 0
    m1_initial = 0
    for ai in a:
        c = max(3 - ai, 0)
        if c == 3:
            m3_initial += 1
        elif c == 2:
            m2_initial += 1
        elif c == 1:
            m1_initial += 1
    
    @lru_cache(maxsize=None)
    def e(m3, m2, m1):
        sum_m = m3 + m2 + m1
        if sum_m == 0:
            return 0.0
        denominator = sum_m / n
        res = 0.0
        if m3 > 0:
            res += (m3 / n) * e(m3 - 1, m2 + 1, m1)
        if m2 > 0:
            res += (m2 / n) * e(m3, m2 - 1, m1 + 1)
        if m1 > 0:
            res += (m1 / n) * e(m3, m2, m1 - 1)
        res = (1 + res) / denominator
        return res
    
    result = e(m3_initial, m2_initial, m1_initial)
    print("{0:.10f}".format(result))

if __name__ == "__main__":
    main()