S=[ord(s)-97 for s in input()]
prev=[-1]*26
N=len(S)
ans=0
for i in range(N):
    prev[S[i]]=i
    bound=[prev[s]+1 for s in range(26) if prev[s]!=-1]+[0]
    bound.sort(reverse=True)
    cnt=0
    for r,l in zip(bound,bound[1:]):
        cnt+=1
        ans+=cnt*(r-l)
ans/=N*(N+1)//2
print(ans)