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