import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();
        int[] arr = new int[200];
        for (int i = 0; i < str.length(); i++) {
            arr[str.charAt(i) - 'A']++;
        }
        int ans = 0;
        int flag = 0;
        for (int i = 0; i < 200; i++) {
            ans += arr[i] / 2 * 2;
            arr[i] %= 2;
            if(arr[i] == 1) flag = 1;
        }
        if(flag == 1) ans++;
        System.out.println(ans);
    }
}