import java.util.*;

public class Main {
	Scanner sc;
	int a,b;
	
	static String testInput = "998244353 99824435";
/*-------------
 * constructor
 */
	public Main() {
		sc = new Scanner(System.in);
	}
	public Main(String input) {
		sc = new Scanner(input);
	}
	
/*------------------
 * instance methods
 */
	void out(Object o) {
		System.out.println(o);
		System.out.flush();
	}
	
	public void exec() throws Exception {
		a = sc.nextInt();
		b = sc.nextInt();
		
		int k = (a+b)/2;
		if (k*2 != a+b) out("IMPOSSIBLE");
		else out(k);
	}
	
/*------
 * main
 */
	public static void main(String[] args) throws Exception {
		Main m = new Main();
		m.exec();
	}
}

	 import java.util.*; util.* java public class Main {
	Scanner sc;
	int a,b;
	
	static String testInput = "998244353 99824435";
/*-------------
 * constructor
 */
	public Main() {
		sc = new Scanner(System.in);
	}
	public Main(String input) {
		sc = new Scanner(input);
	}
	
/*------------------
 * instance methods
 */
	void out(Object o) {
		System.out.println(o);
		System.out.flush();
	}
	
	public void exec() throws Exception {
		a = sc.nextInt();
		b = sc.nextInt();
		
		int k = (a+b)/2;
		if (k*2 != a+b) out("IMPOSSIBLE");
		else out(k);
	}
	
/*------
 * main
 */
	public static void main(String[] args) throws Exception {
		Main m = new Main();
		m.exec();
	}
} Main Scanner sc; sc int a a b; b static String testInput = "998244353 99824435"; testInput "998244353 99824435" /*-------------
 * constructor
 */
	public Main() {
		sc = new Scanner(System.in);
	} Main {
		sc = new Scanner(System.in);
	} sc = new Scanner(System.in) sc new Scanner(System.in) public Main(String input) {
		sc = new Scanner(input);
	} Main {
		sc = new Scanner(input);
	} sc = new Scanner(input) sc new Scanner(input) String input input /*------------------
 * instance methods
 */
	void out(Object o) {
		System.out.println(o);
		System.out.flush();
	} out {
		System.out.println(o);
		System.out.flush();
	} System.out.println(o) System.out.println System.out System System.out o System.out.flush() System.out.flush System.out System System.out Object o o public void exec() throws Exception {
		a = sc.nextInt();
		b = sc.nextInt();
		
		int k = (a+b)/2;
		if (k*2 != a+b) out("IMPOSSIBLE");
		else out(k);
	} exec {
		a = sc.nextInt();
		b = sc.nextInt();
		
		int k = (a+b)/2;
		if (k*2 != a+b) out("IMPOSSIBLE");
		else out(k);
	} a = sc.nextInt() a sc.nextInt() sc.nextInt sc b = sc.nextInt() b sc.nextInt() sc.nextInt sc int k = (a+b)/2; k (a+b)/2 (a+b) a b 2 if (k*2 != a+b) out("IMPOSSIBLE");
		else out(k); k*2 != a+b k*2 k 2 a+b a b out("IMPOSSIBLE") out "IMPOSSIBLE" out(k) out k /*------
 * main
 */
	public static void main(String[] args) throws Exception {
		Main m = new Main();
		m.exec();
	} main {
		Main m = new Main();
		m.exec();
	} Main m = new Main(); m new Main() m.exec() m.exec m String[] args args public class Main {
	Scanner sc;
	int a,b;
	
	static String testInput = "998244353 99824435";
/*-------------
 * constructor
 */
	public Main() {
		sc = new Scanner(System.in);
	}
	public Main(String input) {
		sc = new Scanner(input);
	}
	
/*------------------
 * instance methods
 */
	void out(Object o) {
		System.out.println(o);
		System.out.flush();
	}
	
	public void exec() throws Exception {
		a = sc.nextInt();
		b = sc.nextInt();
		
		int k = (a+b)/2;
		if (k*2 != a+b) out("IMPOSSIBLE");
		else out(k);
	}
	
/*------
 * main
 */
	public static void main(String[] args) throws Exception {
		Main m = new Main();
		m.exec();
	}
} public class Main {
	Scanner sc;
	int a,b;
	
	static String testInput = "998244353 99824435";
/*-------------
 * constructor
 */
	public Main() {
		sc = new Scanner(System.in);
	}
	public Main(String input) {
		sc = new Scanner(input);
	}
	
/*------------------
 * instance methods
 */
	void out(Object o) {
		System.out.println(o);
		System.out.flush();
	}
	
	public void exec() throws Exception {
		a = sc.nextInt();
		b = sc.nextInt();
		
		int k = (a+b)/2;
		if (k*2 != a+b) out("IMPOSSIBLE");
		else out(k);
	}
	
/*------
 * main
 */
	public static void main(String[] args) throws Exception {
		Main m = new Main();
		m.exec();
	}
} Main
import java.util.*;

public class Main {
	Scanner sc;
	int a,b;
	
	static String testInput = "998244353 99824435";
/*-------------
 * constructor
 */
	public Main() {
		sc = new Scanner(System.in);
	}
	public Main(String input) {
		sc = new Scanner(input);
	}
	
/*------------------
 * instance methods
 */
	void out(Object o) {
		System.out.println(o);
		System.out.flush();
	}
	
	public void exec() throws Exception {
		a = sc.nextInt();
		b = sc.nextInt();
		
		int k = (a+b)/2;
		if (k*2 != a+b) out("IMPOSSIBLE");
		else out(k);
	}
	
/*------
 * main
 */
	public static void main(String[] args) throws Exception {
		Main m = new Main();
		m.exec();
	}
}

	