import java.util.*;

public class Main {
public static void main(String[] args) {
		
		Scanner keyboard = new Scanner(System.in); 
		
		double A  = keyboard.nextInt();
		double B  = keyboard.nextInt();
		
		if(((A+B)/2) % 1 == 0.0) {
			System.out.println((int)(A+B)/2);
		}else{
			System.out.println("IMPOSSIBLE");
		}
		keyboard.close();	
	}
} import java.util.*; util.* java public class Main {
public static void main(String[] args) {
		
		Scanner keyboard = new Scanner(System.in); 
		
		double A  = keyboard.nextInt();
		double B  = keyboard.nextInt();
		
		if(((A+B)/2) % 1 == 0.0) {
			System.out.println((int)(A+B)/2);
		}else{
			System.out.println("IMPOSSIBLE");
		}
		keyboard.close();	
	}
} Main public static void main(String[] args) {
		
		Scanner keyboard = new Scanner(System.in); 
		
		double A  = keyboard.nextInt();
		double B  = keyboard.nextInt();
		
		if(((A+B)/2) % 1 == 0.0) {
			System.out.println((int)(A+B)/2);
		}else{
			System.out.println("IMPOSSIBLE");
		}
		keyboard.close();	
	} main {
		
		Scanner keyboard = new Scanner(System.in); 
		
		double A  = keyboard.nextInt();
		double B  = keyboard.nextInt();
		
		if(((A+B)/2) % 1 == 0.0) {
			System.out.println((int)(A+B)/2);
		}else{
			System.out.println("IMPOSSIBLE");
		}
		keyboard.close();	
	} Scanner keyboard = new Scanner(System.in); keyboard new Scanner(System.in) double A  = keyboard.nextInt(); A keyboard.nextInt() keyboard.nextInt keyboard double B  = keyboard.nextInt(); B keyboard.nextInt() keyboard.nextInt keyboard if(((A+B)/2) % 1 == 0.0) {
			System.out.println((int)(A+B)/2);
		}else{
			System.out.println("IMPOSSIBLE");
		} ((A+B)/2) % 1 == 0.0 ((A+B)/2) % 1 ((A+B)/2) (A+B) A B 2 1 0.0 {
			System.out.println((int)(A+B)/2);
		} System.out.println((int)(A+B)/2) System.out.println System.out System System.out (int)(A+B)/2 (int)(A+B) A B 2 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" keyboard.close() keyboard.close keyboard String[] args args public class Main {
public static void main(String[] args) {
		
		Scanner keyboard = new Scanner(System.in); 
		
		double A  = keyboard.nextInt();
		double B  = keyboard.nextInt();
		
		if(((A+B)/2) % 1 == 0.0) {
			System.out.println((int)(A+B)/2);
		}else{
			System.out.println("IMPOSSIBLE");
		}
		keyboard.close();	
	}
} public class Main {
public static void main(String[] args) {
		
		Scanner keyboard = new Scanner(System.in); 
		
		double A  = keyboard.nextInt();
		double B  = keyboard.nextInt();
		
		if(((A+B)/2) % 1 == 0.0) {
			System.out.println((int)(A+B)/2);
		}else{
			System.out.println("IMPOSSIBLE");
		}
		keyboard.close();	
	}
} Main
import java.util.*;

public class Main {
public static void main(String[] args) {
		
		Scanner keyboard = new Scanner(System.in); 
		
		double A  = keyboard.nextInt();
		double B  = keyboard.nextInt();
		
		if(((A+B)/2) % 1 == 0.0) {
			System.out.println((int)(A+B)/2);
		}else{
			System.out.println("IMPOSSIBLE");
		}
		keyboard.close();	
	}
}