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();
      double x = (double) (a+b)/2;
      double y = (a+b)/2;
      if(x == y) 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();
      double x = (double) (a+b)/2;
      double y = (a+b)/2;
      if(x == y) 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();
      double x = (double) (a+b)/2;
      double y = (a+b)/2;
      if(x == y) 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();
      double x = (double) (a+b)/2;
      double y = (a+b)/2;
      if(x == y) 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 double x = (double) (a+b)/2; x (double) (a+b)/2 (double) (a+b) a b 2 double y = (a+b)/2; y (a+b)/2 (a+b) a b 2 if(x == y) System.out.println((a+b)/2);
      else System.out.println("IMPOSSIBLE"); x == y x y 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();
      int b = sc.nextInt();
      double x = (double) (a+b)/2;
      double y = (a+b)/2;
      if(x == y) 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();
      double x = (double) (a+b)/2;
      double y = (a+b)/2;
      if(x == y) 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();
      double x = (double) (a+b)/2;
      double y = (a+b)/2;
      if(x == y) System.out.println((a+b)/2);
      else System.out.println("IMPOSSIBLE");
    }
}