import java.io.*; import java.util.*; import java.util.stream.*; public class Main { public static void main(String[] args) throws Exception { Scanner sc = new Scanner(); char[] inputs = sc.next().toCharArray(); ArrayList> places = new ArrayList<>(); for (int i = 0; i < 26; i++) { places.add(new TreeSet<>()); places.get(i).add(inputs.length); } for (int i = 0; i < inputs.length; i++) { places.get(inputs[i] - 'a').add(i); } double total = 26.0 * inputs.length * (inputs.length + 1) / 2; for (TreeSet tmp : places) { int prev = -1; for (int x : tmp) { long count = x - prev - 1; total -= count * (count + 1) / 2; prev = x; } } total /= inputs.length * (inputs.length + 1L) / 2; System.out.println(total); } } class Utilities { static String arrayToLineString(Object[] arr) { return Arrays.stream(arr).map(x -> x.toString()).collect(Collectors.joining("\n")); } static String arrayToLineString(int[] arr) { return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new)); } } class Scanner { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); StringTokenizer st = new StringTokenizer(""); StringBuilder sb = new StringBuilder(); public Scanner() throws Exception { } public int nextInt() throws Exception { return Integer.parseInt(next()); } public long nextLong() throws Exception { return Long.parseLong(next()); } public double nextDouble() throws Exception { return Double.parseDouble(next()); } public int[] nextIntArray() throws Exception { return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray(); } public String next() throws Exception { while (!st.hasMoreTokens()) { st = new StringTokenizer(br.readLine()); } return st.nextToken(); } }