fun main(args: Array<String>) {
  val rd = Reader()
  val S = rd.string()
  val N = S.length
  val mae = IntArray(26, {-1})
  var kotae = 0L
  for (i in 0..N-1) {
    kotae += (i - mae[S[i] - 'a']).toLong() * (N - i)
    mae[S[i] - 'a'] = i
  }
  println(2.0 * kotae / N / (N + 1))
}

class Reader {
  var buf: ByteArray = ByteArray(5000000)
  var k: Int = 0
  var n: Int = 0

  init {
    check()
  }

  private fun check() {
    if (k == n) {
      n = System.`in`.read(buf)
      k = 0
    }
  }

  private fun peek(): Byte {
    return buf[k]
  }

  private fun next() {
    k++
    check()
  }

  fun string(): String {
    var ans = StringBuilder()
    while ('0'.toByte() > peek()) next()
    while ('0'.toByte() <= peek()) {
      ans.append(peek().toChar())
      next()
    }
    return ans.toString()
  }

  fun long(): Long {
    var ans: Long = 0
    while ('0'.toByte() > peek()) next()
    while ('0'.toByte() <= peek()) {
      ans = ans * 10 + (peek() - '0'.toLong())
      next() 
    }
    return ans
  }

  fun int(): Int {
    return long().toInt()
  }
}