import sequtils,strutils,math,algorithm
var
    S = stdin.readline
    A : array['a'..'z', seq[int]]
    P : array['a'..'z', int]
    ans : float64 = 0.0
    q = 0.0
    
for c in 'a'..'z':
    A[c] = @[]

for i, c in S:
    A[c].add(i)

for i in 0..S.high:
    var B = newSeq[int](0)
    for c in 'a'..'z':
        if A[c].high < P[c]:
            continue
        B.add(A[c][P[c]] - i)
        if c == S[i]:
            P[c] += 1
    for j in 0..B.high:
        ans += (S.high - i + 1 - B[j]).float64
    q += (S.high - i + 1).float64
echo ans / q