import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int sum = -1;
        String str = sc.nextLine();
        str = str.replaceAll("\\D+", " ").trim();
        String[] str1 = str.split(" ");
        int[] arr = new int[str1.length];
        if(str.length()>0) {
            for (int i = 0; i < str1.length; i++) {
                arr[i] = Integer.parseInt(str1[i]);
                if (arr[i] > sum && arr[i] % 2 == 0) {
                    sum = arr[i];
                }
            }
        }
        System.out.println(sum);
    }
}