S = gets.chomp
N = S.size
A = N * (1 + N) / 2
P = Hash.new { |h, k| h[k] = [0] }

N.times do |i|
  s = S[i]
  P[s] << i + 1
end

sum = 0

P.each do |_, positions|
  positions << N + 1
  cnt = 0

  positions.each_cons(2) do |i, j|
    l = j - i - 1
    cnt += l * (1 + l) / 2
  end

  sum += A - cnt
end

puts Rational(sum, A).to_f.round(12)