import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int AplusB = Math.abs(A+B);

        if (AplusB % 2 == 0) {
            System.out.println(AplusB / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int AplusB = Math.abs(A+B);

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

        if (AplusB % 2 == 0) {
            System.out.println(AplusB / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    } main {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int AplusB = Math.abs(A+B);

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

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int AplusB = Math.abs(A+B);

        if (AplusB % 2 == 0) {
            System.out.println(AplusB / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int AplusB = Math.abs(A+B);

        if (AplusB % 2 == 0) {
            System.out.println(AplusB / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int AplusB = Math.abs(A+B);

        if (AplusB % 2 == 0) {
            System.out.println(AplusB / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
}
