S = input()

n = len(S)
total = 0
for i in range(n):
    total += (n - i) * (i + 1)

prev = set()
dic = {}
u = 0
for i in range(n):
    if S[i] in prev:
        idx = dic[S[i]]
        u += (n - i) * (idx + 1)
    prev.add(S[i])
    dic[S[i]] = i

print(round((total - u) / ((n * (n + 1)) // 2), 5))