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();
    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();
    int 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();
    int 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();
    int 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 int 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();
    int 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();
    int 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();
    int b = sc.nextInt();
    System.out.println((a+b)%2 == 0 ? (a+b)/2 : "IMPOSSIBLE");
  }
}