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();
        int ans=(a+b)/2;
    
 	if(a%2==1&&b%2==0){
    		System.out.println("IMPOSSIBLE");
    	}else if(a%2==0&&b%2==1){
     		System.out.println("IMPOSSIBLE");
   	 }else{

    	System.out.println(ans);
    }
    }
} 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();
        int ans=(a+b)/2;
    
 	if(a%2==1&&b%2==0){
    		System.out.println("IMPOSSIBLE");
    	}else if(a%2==0&&b%2==1){
     		System.out.println("IMPOSSIBLE");
   	 }else{

    	System.out.println(ans);
    }
    }
} Main public static void main(String[] args){
	Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        int ans=(a+b)/2;
    
 	if(a%2==1&&b%2==0){
    		System.out.println("IMPOSSIBLE");
    	}else if(a%2==0&&b%2==1){
     		System.out.println("IMPOSSIBLE");
   	 }else{

    	System.out.println(ans);
    }
    } main {
	Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        int ans=(a+b)/2;
    
 	if(a%2==1&&b%2==0){
    		System.out.println("IMPOSSIBLE");
    	}else if(a%2==0&&b%2==1){
     		System.out.println("IMPOSSIBLE");
   	 }else{

    	System.out.println(ans);
    }
    } 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 int ans=(a+b)/2; ans (a+b)/2 (a+b) a b 2 if(a%2==1&&b%2==0){
    		System.out.println("IMPOSSIBLE");
    	}else if(a%2==0&&b%2==1){
     		System.out.println("IMPOSSIBLE");
   	 }else{

    	System.out.println(ans);
    } a%2==1&&b%2==0 a%2==1 a%2 a 2 1 b%2==0 b%2 b 2 0 {
    		System.out.println("IMPOSSIBLE");
    	} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" if(a%2==0&&b%2==1){
     		System.out.println("IMPOSSIBLE");
   	 }else{

    	System.out.println(ans);
    } a%2==0&&b%2==1 a%2==0 a%2 a 2 0 b%2==1 b%2 b 2 1 {
     		System.out.println("IMPOSSIBLE");
   	 } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {

    	System.out.println(ans);
    } System.out.println(ans) System.out.println System.out System System.out ans 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();
        int ans=(a+b)/2;
    
 	if(a%2==1&&b%2==0){
    		System.out.println("IMPOSSIBLE");
    	}else if(a%2==0&&b%2==1){
     		System.out.println("IMPOSSIBLE");
   	 }else{

    	System.out.println(ans);
    }
    }
}
