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();

    if ((Math.abs(b - a) % 2) == 0) {
      System.out.println((Math.abs(b - a) / 2) + Math.min(a, b));
    } 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();

    if ((Math.abs(b - a) % 2) == 0) {
      System.out.println((Math.abs(b - a) / 2) + Math.min(a, b));
    } 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();

    if ((Math.abs(b - a) % 2) == 0) {
      System.out.println((Math.abs(b - a) / 2) + Math.min(a, b));
    } else {
      System.out.println("IMPOSSIBLE");
    }

  } main {

    Scanner sc = new Scanner(System.in);

    int a = sc.nextInt();
    int b = sc.nextInt();

    if ((Math.abs(b - a) % 2) == 0) {
      System.out.println((Math.abs(b - a) / 2) + Math.min(a, b));
    } 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 if ((Math.abs(b - a) % 2) == 0) {
      System.out.println((Math.abs(b - a) / 2) + Math.min(a, b));
    } else {
      System.out.println("IMPOSSIBLE");
    } (Math.abs(b - a) % 2) == 0 (Math.abs(b - a) % 2) Math.abs(b - a) Math.abs Math b - a b a 2 0 {
      System.out.println((Math.abs(b - a) / 2) + Math.min(a, b));
    } System.out.println((Math.abs(b - a) / 2) + Math.min(a, b)) System.out.println System.out System System.out (Math.abs(b - a) / 2) + Math.min(a, b) (Math.abs(b - a) / 2) Math.abs(b - a) Math.abs Math b - a b a 2 Math.min(a, b) Math.min Math a b {
      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();

    if ((Math.abs(b - a) % 2) == 0) {
      System.out.println((Math.abs(b - a) / 2) + Math.min(a, b));
    } 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();

    if ((Math.abs(b - a) % 2) == 0) {
      System.out.println((Math.abs(b - a) / 2) + Math.min(a, b));
    } 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();

    if ((Math.abs(b - a) % 2) == 0) {
      System.out.println((Math.abs(b - a) / 2) + Math.min(a, b));
    } else {
      System.out.println("IMPOSSIBLE");
    }

  }
}