
import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
      String[] nyu=sc.nextLine().split(" ");
   //  String nyu=sc.nextLine();


int num=Integer.parseInt(nyu[0]);
int numnext=Integer.parseInt(nyu[1]);


int count=(num+numnext)/2;
if(count*2==(num+numnext)){
System.out.println(count);

}else{
System.out.println("IMPOSSIBLE");

}



//System.out.println(count);

//System.out.println(a/b);


	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
      String[] nyu=sc.nextLine().split(" ");
   //  String nyu=sc.nextLine();


int num=Integer.parseInt(nyu[0]);
int numnext=Integer.parseInt(nyu[1]);


int count=(num+numnext)/2;
if(count*2==(num+numnext)){
System.out.println(count);

}else{
System.out.println("IMPOSSIBLE");

}



//System.out.println(count);

//System.out.println(a/b);


	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
      String[] nyu=sc.nextLine().split(" ");
   //  String nyu=sc.nextLine();


int num=Integer.parseInt(nyu[0]);
int numnext=Integer.parseInt(nyu[1]);


int count=(num+numnext)/2;
if(count*2==(num+numnext)){
System.out.println(count);

}else{
System.out.println("IMPOSSIBLE");

}



//System.out.println(count);

//System.out.println(a/b);


	} main {
		Scanner sc = new Scanner(System.in);
      String[] nyu=sc.nextLine().split(" ");
   //  String nyu=sc.nextLine();


int num=Integer.parseInt(nyu[0]);
int numnext=Integer.parseInt(nyu[1]);


int count=(num+numnext)/2;
if(count*2==(num+numnext)){
System.out.println(count);

}else{
System.out.println("IMPOSSIBLE");

}



//System.out.println(count);

//System.out.println(a/b);


	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String[] nyu=sc.nextLine().split(" "); nyu sc.nextLine().split(" ") sc.nextLine().split sc.nextLine() sc.nextLine sc " " int num=Integer.parseInt(nyu[0]); num Integer.parseInt(nyu[0]) Integer.parseInt Integer nyu[0] nyu 0 int numnext=Integer.parseInt(nyu[1]); numnext Integer.parseInt(nyu[1]) Integer.parseInt Integer nyu[1] nyu 1 int count=(num+numnext)/2; count (num+numnext)/2 (num+numnext) num numnext 2 if(count*2==(num+numnext)){
System.out.println(count);

}else{
System.out.println("IMPOSSIBLE");

} count*2==(num+numnext) count*2 count 2 (num+numnext) num numnext {
System.out.println(count);

} System.out.println(count) System.out.println System.out System System.out count {
System.out.println("IMPOSSIBLE");

} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
      String[] nyu=sc.nextLine().split(" ");
   //  String nyu=sc.nextLine();


int num=Integer.parseInt(nyu[0]);
int numnext=Integer.parseInt(nyu[1]);


int count=(num+numnext)/2;
if(count*2==(num+numnext)){
System.out.println(count);

}else{
System.out.println("IMPOSSIBLE");

}



//System.out.println(count);

//System.out.println(a/b);


	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
      String[] nyu=sc.nextLine().split(" ");
   //  String nyu=sc.nextLine();


int num=Integer.parseInt(nyu[0]);
int numnext=Integer.parseInt(nyu[1]);


int count=(num+numnext)/2;
if(count*2==(num+numnext)){
System.out.println(count);

}else{
System.out.println("IMPOSSIBLE");

}



//System.out.println(count);

//System.out.println(a/b);


	}
} Main

import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
      String[] nyu=sc.nextLine().split(" ");
   //  String nyu=sc.nextLine();


int num=Integer.parseInt(nyu[0]);
int numnext=Integer.parseInt(nyu[1]);


int count=(num+numnext)/2;
if(count*2==(num+numnext)){
System.out.println(count);

}else{
System.out.println("IMPOSSIBLE");

}



//System.out.println(count);

//System.out.println(a/b);


	}
}