import java.util.Scanner;

public class Main {
    public static int A = 0;
    public static int B = 0;

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        A = sc.nextInt();
        B = sc.nextInt();

        String output = "";
        int C = A + Math.abs(B);
        if (C % 2 == 0) {
            output = Integer.toString(C / 2);
        } else {
            output = "IMPOSSIBLE";
        }
        System.out.println(output);
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static int A = 0;
    public static int B = 0;

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        A = sc.nextInt();
        B = sc.nextInt();

        String output = "";
        int C = A + Math.abs(B);
        if (C % 2 == 0) {
            output = Integer.toString(C / 2);
        } else {
            output = "IMPOSSIBLE";
        }
        System.out.println(output);
    }
} Main public static int A = 0; A 0 public static int B = 0; B 0 public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        A = sc.nextInt();
        B = sc.nextInt();

        String output = "";
        int C = A + Math.abs(B);
        if (C % 2 == 0) {
            output = Integer.toString(C / 2);
        } else {
            output = "IMPOSSIBLE";
        }
        System.out.println(output);
    } main {

        Scanner sc = new Scanner(System.in);
        A = sc.nextInt();
        B = sc.nextInt();

        String output = "";
        int C = A + Math.abs(B);
        if (C % 2 == 0) {
            output = Integer.toString(C / 2);
        } else {
            output = "IMPOSSIBLE";
        }
        System.out.println(output);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) A = sc.nextInt() A sc.nextInt() sc.nextInt sc B = sc.nextInt() B sc.nextInt() sc.nextInt sc String output = ""; output "" int C = A + Math.abs(B); C A + Math.abs(B) A Math.abs(B) Math.abs Math B if (C % 2 == 0) {
            output = Integer.toString(C / 2);
        } else {
            output = "IMPOSSIBLE";
        } C % 2 == 0 C % 2 C 2 0 {
            output = Integer.toString(C / 2);
        } output = Integer.toString(C / 2) output Integer.toString(C / 2) Integer.toString Integer C / 2 C 2 {
            output = "IMPOSSIBLE";
        } output = "IMPOSSIBLE" output "IMPOSSIBLE" System.out.println(output) System.out.println System.out System System.out output String[] args args public class Main {
    public static int A = 0;
    public static int B = 0;

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        A = sc.nextInt();
        B = sc.nextInt();

        String output = "";
        int C = A + Math.abs(B);
        if (C % 2 == 0) {
            output = Integer.toString(C / 2);
        } else {
            output = "IMPOSSIBLE";
        }
        System.out.println(output);
    }
} public class Main {
    public static int A = 0;
    public static int B = 0;

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        A = sc.nextInt();
        B = sc.nextInt();

        String output = "";
        int C = A + Math.abs(B);
        if (C % 2 == 0) {
            output = Integer.toString(C / 2);
        } else {
            output = "IMPOSSIBLE";
        }
        System.out.println(output);
    }
} Main
import java.util.Scanner;

public class Main {
    public static int A = 0;
    public static int B = 0;

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        A = sc.nextInt();
        B = sc.nextInt();

        String output = "";
        int C = A + Math.abs(B);
        if (C % 2 == 0) {
            output = Integer.toString(C / 2);
        } else {
            output = "IMPOSSIBLE";
        }
        System.out.println(output);
    }
}