def main():
    import sys
    S = sys.stdin.readline().strip()
    n = len(S)
    prev = [-1] * 26
    total = 0
    for i in range(n):
        c = S[i]
        idx = ord(c) - ord('a')
        prev_pos = prev[idx]
        add = (i - prev_pos) * (n - i)
        total += add
        prev[idx] = i
    denominator = n * (n + 1) // 2
    average = total / denominator
    print("{0:.5f}".format(average))

if __name__ == "__main__":
    main()