S = input()
N = len(S)

# 左からi文字目移行の文字数 [i, N]
maxSize = [0] * (N + 1)
V = set()
for i, s in enumerate(S[:: -1], start=2):
    V.add(s)
    maxSize[-i] = len(V)

ans = 0
for left, s in enumerate(S):
    V = set()
    for right, t in enumerate(S[left:], start=left):
        V.add(t)
        if len(V) == maxSize[left]:
            ans += len(V) * (N - right)
            break
        ans += len(V)

print(ans / (N * (N + 1) // 2))