import java.util.*;
class Main {
	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
      if(A==0&&B==0){System.out.println("IMPOSSIBLE");}else{
      	if((A+B)%2==0){
		int K = (A+B)/2;
		if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			}
        }else{
          System.out.println("IMPOSSIBLE");
        }
        }
	}
} import java.util.*; util.* java class Main {
	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
      if(A==0&&B==0){System.out.println("IMPOSSIBLE");}else{
      	if((A+B)%2==0){
		int K = (A+B)/2;
		if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			}
        }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();
      if(A==0&&B==0){System.out.println("IMPOSSIBLE");}else{
      	if((A+B)%2==0){
		int K = (A+B)/2;
		if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			}
        }else{
          System.out.println("IMPOSSIBLE");
        }
        }
	} main {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
      if(A==0&&B==0){System.out.println("IMPOSSIBLE");}else{
      	if((A+B)%2==0){
		int K = (A+B)/2;
		if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			}
        }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 if(A==0&&B==0){System.out.println("IMPOSSIBLE");}else{
      	if((A+B)%2==0){
		int K = (A+B)/2;
		if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			}
        }else{
          System.out.println("IMPOSSIBLE");
        }
        } A==0&&B==0 A==0 A 0 B==0 B 0 {System.out.println("IMPOSSIBLE");} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
      	if((A+B)%2==0){
		int K = (A+B)/2;
		if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			}
        }else{
          System.out.println("IMPOSSIBLE");
        }
        } if((A+B)%2==0){
		int K = (A+B)/2;
		if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			}
        }else{
          System.out.println("IMPOSSIBLE");
        } (A+B)%2==0 (A+B)%2 (A+B) A B 2 0 {
		int K = (A+B)/2;
		if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			}
        } int K = (A+B)/2; K (A+B)/2 (A+B) A B 2 if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			} (A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0) (A-K>=0&&B-K<=0) A-K>=0 A-K A K 0 B-K<=0 B-K B K 0 (B-K>=0&&A-K<=0) B-K>=0 B-K B K 0 A-K<=0 A-K A K 0 {
			System.out.println(K);} System.out.println(K) System.out.println System.out System System.out K {
				System.out.println("IMPOSSIBLE");
			} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
          System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args
import java.util.*;
class Main {
	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
      if(A==0&&B==0){System.out.println("IMPOSSIBLE");}else{
      	if((A+B)%2==0){
		int K = (A+B)/2;
		if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			}
        }else{
          System.out.println("IMPOSSIBLE");
        }
        }
	}
}