import java.util.Scanner;
public class Main{
	static int getK(int a,int b) {  
		int i,k=0;
		  i=a+b;
		    k=i/2;
		  return k ;
  } 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int a,b,i,k=1;
	Scanner scan=new Scanner(System.in);
	 a=scan.nextInt();
	 b=scan.nextInt();
    i=a+b;
    if(i%2==0) {
    System.out.println(getK(a,b));
   }
   else { 
   System.out.println("IMPOSSIBLE");
   }
	   }
	
	 } import java.util.Scanner; Scanner java.util public class Main{
	static int getK(int a,int b) {  
		int i,k=0;
		  i=a+b;
		    k=i/2;
		  return k ;
  } 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int a,b,i,k=1;
	Scanner scan=new Scanner(System.in);
	 a=scan.nextInt();
	 b=scan.nextInt();
    i=a+b;
    if(i%2==0) {
    System.out.println(getK(a,b));
   }
   else { 
   System.out.println("IMPOSSIBLE");
   }
	   }
	
	 } Main static int getK(int a,int b) {  
		int i,k=0;
		  i=a+b;
		    k=i/2;
		  return k ;
  } getK {  
		int i,k=0;
		  i=a+b;
		    k=i/2;
		  return k ;
  } int i i k=0; k 0 i=a+b i a+b a b k=i/2 k i/2 i 2 return k ; k int a a int b b public static void main(String[] args) {
		// TODO Auto-generated method stub
		int a,b,i,k=1;
	Scanner scan=new Scanner(System.in);
	 a=scan.nextInt();
	 b=scan.nextInt();
    i=a+b;
    if(i%2==0) {
    System.out.println(getK(a,b));
   }
   else { 
   System.out.println("IMPOSSIBLE");
   }
	   } main {
		// TODO Auto-generated method stub
		int a,b,i,k=1;
	Scanner scan=new Scanner(System.in);
	 a=scan.nextInt();
	 b=scan.nextInt();
    i=a+b;
    if(i%2==0) {
    System.out.println(getK(a,b));
   }
   else { 
   System.out.println("IMPOSSIBLE");
   }
	   } int a a b b i i k=1; k 1 Scanner scan=new Scanner(System.in); scan new Scanner(System.in) a=scan.nextInt() a scan.nextInt() scan.nextInt scan b=scan.nextInt() b scan.nextInt() scan.nextInt scan i=a+b i a+b a b if(i%2==0) {
    System.out.println(getK(a,b));
   }
   else { 
   System.out.println("IMPOSSIBLE");
   } i%2==0 i%2 i 2 0 {
    System.out.println(getK(a,b));
   } System.out.println(getK(a,b)) System.out.println System.out System System.out getK(a,b) getK a b { 
   System.out.println("IMPOSSIBLE");
   } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args public class Main{
	static int getK(int a,int b) {  
		int i,k=0;
		  i=a+b;
		    k=i/2;
		  return k ;
  } 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int a,b,i,k=1;
	Scanner scan=new Scanner(System.in);
	 a=scan.nextInt();
	 b=scan.nextInt();
    i=a+b;
    if(i%2==0) {
    System.out.println(getK(a,b));
   }
   else { 
   System.out.println("IMPOSSIBLE");
   }
	   }
	
	 } public class Main{
	static int getK(int a,int b) {  
		int i,k=0;
		  i=a+b;
		    k=i/2;
		  return k ;
  } 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int a,b,i,k=1;
	Scanner scan=new Scanner(System.in);
	 a=scan.nextInt();
	 b=scan.nextInt();
    i=a+b;
    if(i%2==0) {
    System.out.println(getK(a,b));
   }
   else { 
   System.out.println("IMPOSSIBLE");
   }
	   }
	
	 } Main
import java.util.Scanner;
public class Main{
	static int getK(int a,int b) {  
		int i,k=0;
		  i=a+b;
		    k=i/2;
		  return k ;
  } 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int a,b,i,k=1;
	Scanner scan=new Scanner(System.in);
	 a=scan.nextInt();
	 b=scan.nextInt();
    i=a+b;
    if(i%2==0) {
    System.out.println(getK(a,b));
   }
   else { 
   System.out.println("IMPOSSIBLE");
   }
	   }
	
	 }