import java.util.Scanner;

public class PrimeNumbers {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        String input = scan.nextLine().replace(".", " ");
        String[] arr = input.split(" ");

        int maxNum = -1;
        int num = -1;
        boolean isNum = true;
        for (int i = 0; i < arr.length; i++) {
            try {
                num = Integer.parseInt(arr[i]);
            } catch (NumberFormatException e) {
                isNum = false;
            }
            if (isNum) {
                num = Integer.parseInt(arr[i]);
            }
            if (num % 2 == 0 && num > maxNum) {
                maxNum = num;
            }
            isNum = true;
        }
        if (maxNum != -1) {
            System.out.println(maxNum);
        } else {
            System.out.println("-1");
        }
    }
}