s=input()
l=len(s)
t=set()
v=[0]
tmp=0
for i in range(l):
    if s[-(i+1)] not in t:
        t.add(s[-(i+1)])
        tmp+=1
    v.append(tmp)
ans=0
for i in range(1,l+1):
    ans+=v[i]*(l-i+1)
ans/=(l*(l+1))//2
print(ans)