
import java.util.Scanner;
public class Main{
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int a = Integer.parseInt(scan.next());
        int b = Integer.parseInt(scan.next());
 
        scan.close();
      
      if((a+b)%2 == 0){
        System.out.println(""+(a+b)/2);
      }else{
        System.out.println("IMPOSSIBLE");
      }
    }
} import java.util.Scanner; Scanner java.util public class Main{
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int a = Integer.parseInt(scan.next());
        int b = Integer.parseInt(scan.next());
 
        scan.close();
      
      if((a+b)%2 == 0){
        System.out.println(""+(a+b)/2);
      }else{
        System.out.println("IMPOSSIBLE");
      }
    }
} Main public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int a = Integer.parseInt(scan.next());
        int b = Integer.parseInt(scan.next());
 
        scan.close();
      
      if((a+b)%2 == 0){
        System.out.println(""+(a+b)/2);
      }else{
        System.out.println("IMPOSSIBLE");
      }
    } main {
        Scanner scan = new Scanner(System.in);
        int a = Integer.parseInt(scan.next());
        int b = Integer.parseInt(scan.next());
 
        scan.close();
      
      if((a+b)%2 == 0){
        System.out.println(""+(a+b)/2);
      }else{
        System.out.println("IMPOSSIBLE");
      }
    } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int a = Integer.parseInt(scan.next()); a Integer.parseInt(scan.next()) Integer.parseInt Integer scan.next() scan.next scan int b = Integer.parseInt(scan.next()); b Integer.parseInt(scan.next()) Integer.parseInt Integer scan.next() scan.next scan scan.close() scan.close scan 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)/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 scan = new Scanner(System.in);
        int a = Integer.parseInt(scan.next());
        int b = Integer.parseInt(scan.next());
 
        scan.close();
      
      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 scan = new Scanner(System.in);
        int a = Integer.parseInt(scan.next());
        int b = Integer.parseInt(scan.next());
 
        scan.close();
      
      if((a+b)%2 == 0){
        System.out.println(""+(a+b)/2);
      }else{
        System.out.println("IMPOSSIBLE");
      }
    }
} Main

import java.util.Scanner;
public class Main{
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int a = Integer.parseInt(scan.next());
        int b = Integer.parseInt(scan.next());
 
        scan.close();
      
      if((a+b)%2 == 0){
        System.out.println(""+(a+b)/2);
      }else{
        System.out.println("IMPOSSIBLE");
      }
    }
}