S = list(input())
N = len(S)
alpha = "abcdefghijklmnopqrstuvwxyz"
d = {alpha[i]: [0] for i in range(26)}
for i in range(N):
    d[S[i]].append(i+1)
for i in range(26):
    d[alpha[i]].append(N+1)
ans = 0
down = N*(N+1)//2
for i in range(26):
    P = N*(N+1)//2
    for j in range(len(d[alpha[i]])-1):
        x, y = d[alpha[i]][j], d[alpha[i]][j+1]
        P -= (y-x)*(y-x-1)//2
    ans += P/down
print(ans)