class Bit:
    def __init__(self, n):
        self.size = n
        self.tree = [0] * (n + 1)
 
    def sum(self, i):
        s = 0
        while i > 0:
            s += self.tree[i]
            i -= i & -i
        return s
 
    def add(self, i, x):
        while i <= self.size:
            self.tree[i] += x
            i += i & -i

class RangeUpdate:
    def __init__(self, n):
        self.p = Bit(n + 1)
        self.q = Bit(n + 1)
     
    def add(self, s, t, x):
        t += 1
        self.p.add(s, -x * s)
        self.p.add(t, x * t)
        self.q.add(s, x)
        self.q.add(t, -x)
     
    def sum(self, s, t):
        t += 1
        return self.p.sum(t) + self.q.sum(t) * t - \
               self.p.sum(s) - self.q.sum(s) * s
    #yukicoder218B

s = input()
ngs = len(s)

alp = [[] for i in range(26)]
li = [0]*(ngs)
rui = [0]*(ngs+1)

for i in range(ngs):
    idx = ord(s[i]) - ord("a")
    li[i] = li[i-1]
    if len(alp[idx]) == 0:
        li[i] += 1
    alp[idx].append(i)

ru = RangeUpdate(ngs+1)

for i in range(ngs):
    ru.add(i+1,i+1,li[i])

ans = ru.sum(1,ngs)
aidx = [1]*26
for i in range(1,ngs):
    idx = ord(s[i-1]) - ord("a")
    if len(alp[idx]) <= aidx[idx]:
        ru.add(i+1,ngs,-1)
    else:
        ru.add(i+1,alp[idx][aidx[idx]],-1)
        aidx[idx] += 1
    ans += ru.sum(i+1,ngs)
print(ans/((ngs*(ngs+1))/2))