import java.util.*;
public class Main {
static long A= 0;
static long B =0;
static long C=0;

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		A = sc.nextInt();
		B=  sc.nextInt();
		C=0;
		
		for(int i=1;i<Math.max(A,B);i++) {
			if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			}
		}
		if(C==0) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(C);
		}
		

	}
} import java.util.*; util.* java public class Main {
static long A= 0;
static long B =0;
static long C=0;

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		A = sc.nextInt();
		B=  sc.nextInt();
		C=0;
		
		for(int i=1;i<Math.max(A,B);i++) {
			if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			}
		}
		if(C==0) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(C);
		}
		

	}
} Main static long A= 0; A 0 static long B =0; B 0 static long C=0; C 0 public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		A = sc.nextInt();
		B=  sc.nextInt();
		C=0;
		
		for(int i=1;i<Math.max(A,B);i++) {
			if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			}
		}
		if(C==0) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(C);
		}
		

	} main {
		Scanner sc = new Scanner(System.in);
		A = sc.nextInt();
		B=  sc.nextInt();
		C=0;
		
		for(int i=1;i<Math.max(A,B);i++) {
			if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			}
		}
		if(C==0) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(C);
		}
		

	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) A = sc.nextInt() A sc.nextInt() sc.nextInt sc B=  sc.nextInt() B sc.nextInt() sc.nextInt sc C=0 C 0 for(int i=1;i<Math.max(A,B);i++) {
			if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			}
		} int i=1; int i=1; i 1 i<Math.max(A,B) i Math.max(A,B) Math.max Math A B i++ i++ i {
			if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			}
		} {
			if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			}
		} if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			} Math.abs(i-A)==Math.abs(i-B) Math.abs(i-A) Math.abs Math i-A i A Math.abs(i-B) Math.abs Math i-B i B {
				C=i;
				break;
			} C=i C i break; if(C==0) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(C);
		} C==0 C 0 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			System.out.println(C);
		} System.out.println(C) System.out.println System.out System System.out C String args[] args public class Main {
static long A= 0;
static long B =0;
static long C=0;

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		A = sc.nextInt();
		B=  sc.nextInt();
		C=0;
		
		for(int i=1;i<Math.max(A,B);i++) {
			if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			}
		}
		if(C==0) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(C);
		}
		

	}
} public class Main {
static long A= 0;
static long B =0;
static long C=0;

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		A = sc.nextInt();
		B=  sc.nextInt();
		C=0;
		
		for(int i=1;i<Math.max(A,B);i++) {
			if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			}
		}
		if(C==0) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(C);
		}
		

	}
} Main
import java.util.*;
public class Main {
static long A= 0;
static long B =0;
static long C=0;

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		A = sc.nextInt();
		B=  sc.nextInt();
		C=0;
		
		for(int i=1;i<Math.max(A,B);i++) {
			if(Math.abs(i-A)==Math.abs(i-B)) {
				C=i;
				break;
			}
		}
		if(C==0) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(C);
		}
		

	}
}
