S=input()
L=len(S)

M=[[1<<40]*(L+1) for i in range(26)]

for i in range(L-1,-1,-1):
    for j in range(26):
        if ord(S[i])-97==j:
            M[j][i]=i
        else:
            M[j][i]=M[j][i+1]

ANS=0

for j in range(26):
    for i in range(L):
        if M[j][i]==1<<40:
            continue
        ANS+=L-M[j][i]

print(ANS/(L*(L+1)/2))