#yuki852
A='abcdefghijklmnopqrstuvwxyz'
s=input()
pos={}
for c in A:
 pos[c]=-1
res=0
dp=0
for i in range(len(s)):
 dp+=i-pos[s[i]]
 res+=dp
 pos[s[i]]=i
res/=(len(s)*(len(s)+1)//2)
print(res)