from collections import defaultdict

N = int(input())
P = [int(x) for x in input().strip().split()]
SP = sorted(P)
if P == SP:
    print(0)
else:
    facs = [1]
    for i in range(1, N+1):
        facs.append(facs[-1] * i)
    cnt = defaultdict(int)
    for x in P:
        cnt[x] += 1
    acc = facs[N]
    for k, v in cnt.items():
        acc /= facs[v]
    print("%.6f" % acc)