s = str(input())

n = len(s)
ans = 0
for i in range(26):
    c = chr(i+ord('a'))
    X = []
    cnt = 0
    for j in range(n):
        if s[j] != c:
            cnt += 1
        else:
            X.append(cnt)
            cnt = 0
    else:
        X.append(cnt)
    temp = n*(n+1)//2
    for x in X:
        temp -= x*(x+1)//2
    ans += temp
ans /= n*(n+1)//2
print(ans)