using System;
using System.Collections.Generic;
using System.Linq;

class Solution
{
    static void Main()
    {
        var s = Console.ReadLine();
        var counts = new Dictionary<char, int>();
        for (char c = 'a'; c <= 'z'; c++)
        {
            counts[c] = 0;
        }
        foreach (var c in s)
        {
            counts[c]++;
        }

        var n = Math.Max(1, counts.Values.Count(c => c % 2 != 0));
        Console.WriteLine(n);
    }
}