import java.util.Scanner;

public class Main {

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

    long A = Long.parseLong(sc.next());
    long B = Long.parseLong(sc.next());

    long n = Math.abs(A - (A-B) / 2);

    if (Math.abs(A-n) == Math.abs(B-n)) {
      System.out.println(n);
    } 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);

    long A = Long.parseLong(sc.next());
    long B = Long.parseLong(sc.next());

    long n = Math.abs(A - (A-B) / 2);

    if (Math.abs(A-n) == Math.abs(B-n)) {
      System.out.println(n);
    } else {
      System.out.println("IMPOSSIBLE");
    }
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);

    long A = Long.parseLong(sc.next());
    long B = Long.parseLong(sc.next());

    long n = Math.abs(A - (A-B) / 2);

    if (Math.abs(A-n) == Math.abs(B-n)) {
      System.out.println(n);
    } else {
      System.out.println("IMPOSSIBLE");
    }
  } main {
    Scanner sc = new Scanner(System.in);

    long A = Long.parseLong(sc.next());
    long B = Long.parseLong(sc.next());

    long n = Math.abs(A - (A-B) / 2);

    if (Math.abs(A-n) == Math.abs(B-n)) {
      System.out.println(n);
    } else {
      System.out.println("IMPOSSIBLE");
    }
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long A = Long.parseLong(sc.next()); A Long.parseLong(sc.next()) Long.parseLong Long sc.next() sc.next sc long B = Long.parseLong(sc.next()); B Long.parseLong(sc.next()) Long.parseLong Long sc.next() sc.next sc long n = Math.abs(A - (A-B) / 2); n Math.abs(A - (A-B) / 2) Math.abs Math A - (A-B) / 2 A (A-B) / 2 (A-B) A B 2 if (Math.abs(A-n) == Math.abs(B-n)) {
      System.out.println(n);
    } else {
      System.out.println("IMPOSSIBLE");
    } Math.abs(A-n) == Math.abs(B-n) Math.abs(A-n) Math.abs Math A-n A n Math.abs(B-n) Math.abs Math B-n B n {
      System.out.println(n);
    } System.out.println(n) System.out.println System.out System System.out n {
      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);

    long A = Long.parseLong(sc.next());
    long B = Long.parseLong(sc.next());

    long n = Math.abs(A - (A-B) / 2);

    if (Math.abs(A-n) == Math.abs(B-n)) {
      System.out.println(n);
    } else {
      System.out.println("IMPOSSIBLE");
    }
  }
} public class Main {

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

    long A = Long.parseLong(sc.next());
    long B = Long.parseLong(sc.next());

    long n = Math.abs(A - (A-B) / 2);

    if (Math.abs(A-n) == Math.abs(B-n)) {
      System.out.println(n);
    } 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);

    long A = Long.parseLong(sc.next());
    long B = Long.parseLong(sc.next());

    long n = Math.abs(A - (A-B) / 2);

    if (Math.abs(A-n) == Math.abs(B-n)) {
      System.out.println(n);
    } else {
      System.out.println("IMPOSSIBLE");
    }
  }
}