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