fun main() {
    val builder = StringBuilder()

    val s = readInputLine()

    val posArr = Array('z' - 'a' + 1) { mutableListOf<Int>() }

    val allPat = s.length.toLong() * (s.length + 1).toLong() / 2L

    s.forEachIndexed { index, c ->
        posArr[c - 'a'].add(index)
    }

    var cnt = 0L

    for (pos in posArr) {
        var adder = allPat
        var prevIndex = -1
        for (i in pos) {
            // prevIndex + 1 ～ i - 1の範囲で組む部分文字列パターン数を除外
            adder -= (i - prevIndex - 1).toLong() * (i - prevIndex).toLong() / 2L
            prevIndex = i
        }
        adder -= (s.length - prevIndex - 1).toLong() * (s.length - prevIndex).toLong() / 2L

        cnt += adder
    }

    builder.appendln(cnt.toDouble() / allPat.toDouble())

    print(builder.toString())
}

fun readInputLine(): String {
    return readLine()!!
}