local s = io.read()
local n = #s
local t = {}
for i = 1, 26 do
  t[i] = {}
end
for i = 1, n do
  local b = s:sub(i, i):byte() - 96
  table.insert(t[b], i)
end
-- inv sort
for i = 1, 26 do
  local tn = #t[i]
  local htn = math.floor(tn / 2)
  for j = 1, htn do
    t[i][j], t[i][tn + 1 - j] = t[i][tn + 1 - j], t[i][j]
  end
end
local val = 0
for i = 1, 26 do
  if 0 < #t[i] then
    val = val + n + 1 - t[i][#t[i]]
  end
end
local cnt = 0
for i = 1, n do
  cnt = cnt + val
  local b = s:sub(i, i):byte() - 96
  if t[b][#t[b]] == i then
    val = val - (n + 1 - t[b][#t[b]])
    table.remove(t[b])
    if 0 < #t[b] then
      val = val + (n + 1 - t[b][#t[b]])
    end
  end
end
local all = n * (n + 1) / 2
print(string.format("%.10f", cnt / all))