using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static long NN => long.Parse(ReadLine());
    static long[] NList => ReadLine().Split().Select(long.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var s = ReadLine();
        var dp = Enumerable.Repeat(s.Length, 27).ToArray();
        var arr = new int[27];
        var sum = 0L;
        for (var i = s.Length - 1; i >= 0; --i)
        {
            dp[s[i] - 'a'] = i;
            Array.Copy(dp, arr, 27);
            Array.Sort(arr);
            for (var j = 1; j < dp.Length; ++j) sum += (dp[j] - dp[j - 1]) * j;
        }
        WriteLine(sum * 2.0 / s.Length / (s.Length + 1));
    }
}