import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt(), B = sc.nextInt();
    System.out.println(((A - B) % 2 == 0) ? (A + B) / 2 : "IMPOSSIBLE");
  }
} import java.util.*; util.* java public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt(), B = sc.nextInt();
    System.out.println(((A - B) % 2 == 0) ? (A + B) / 2 : "IMPOSSIBLE");
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt(), B = sc.nextInt();
    System.out.println(((A - B) % 2 == 0) ? (A + B) / 2 : "IMPOSSIBLE");
  } main {
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt(), B = sc.nextInt();
    System.out.println(((A - B) % 2 == 0) ? (A + B) / 2 : "IMPOSSIBLE");
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int A = sc.nextInt() A sc.nextInt() sc.nextInt sc B = sc.nextInt(); B sc.nextInt() sc.nextInt sc System.out.println(((A - B) % 2 == 0) ? (A + B) / 2 : "IMPOSSIBLE") System.out.println System.out System System.out ((A - B) % 2 == 0) ? (A + B) / 2 : "IMPOSSIBLE" ((A - B) % 2 == 0) (A - B) % 2 (A - B) A B 2 0 (A + B) / 2 (A + B) A B 2 "IMPOSSIBLE" String[] args args public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt(), B = sc.nextInt();
    System.out.println(((A - B) % 2 == 0) ? (A + B) / 2 : "IMPOSSIBLE");
  }
} public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt(), B = sc.nextInt();
    System.out.println(((A - B) % 2 == 0) ? (A + B) / 2 : "IMPOSSIBLE");
  }
} Main
import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt(), B = sc.nextInt();
    System.out.println(((A - B) % 2 == 0) ? (A + B) / 2 : "IMPOSSIBLE");
  }
}