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%2 != b%2) System.out.println("IMPOSSIBLE");
		else System.out.println((a+b)/2);
    }
} 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%2 != b%2) System.out.println("IMPOSSIBLE");
		else System.out.println((a+b)/2);
    }
} Main public static void main(String[] args){
    	Scanner sc = new Scanner(System.in);
      	int a = sc.nextInt();
		int b = sc.nextInt();
      	if(a%2 != b%2) System.out.println("IMPOSSIBLE");
		else System.out.println((a+b)/2);
    } main {
    	Scanner sc = new Scanner(System.in);
      	int a = sc.nextInt();
		int b = sc.nextInt();
      	if(a%2 != b%2) System.out.println("IMPOSSIBLE");
		else System.out.println((a+b)/2);
    } 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%2 != b%2) System.out.println("IMPOSSIBLE");
		else System.out.println((a+b)/2); a%2 != b%2 a%2 a 2 b%2 b 2 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" System.out.println((a+b)/2) System.out.println System.out System System.out (a+b)/2 (a+b) a b 2 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%2 != b%2) System.out.println("IMPOSSIBLE");
		else System.out.println((a+b)/2);
    }
} 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%2 != b%2) System.out.println("IMPOSSIBLE");
		else System.out.println((a+b)/2);
    }
} 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%2 != b%2) System.out.println("IMPOSSIBLE");
		else System.out.println((a+b)/2);
    }
}