/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = Math.abs(b+a);
		if(c%2!=0) System.out.println("IMPOSSIBLE");
		else System.out.println(c/2);
	}
} import java.util.*; util.* java import java.lang.*; lang.* java import java.io.*; io.* java public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = Math.abs(b+a);
		if(c%2!=0) System.out.println("IMPOSSIBLE");
		else System.out.println(c/2);
	}
} Main public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = Math.abs(b+a);
		if(c%2!=0) System.out.println("IMPOSSIBLE");
		else System.out.println(c/2);
	} main {
		// your code goes here
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = Math.abs(b+a);
		if(c%2!=0) System.out.println("IMPOSSIBLE");
		else System.out.println(c/2);
	} 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 c = Math.abs(b+a); c Math.abs(b+a) Math.abs Math b+a b a if(c%2!=0) System.out.println("IMPOSSIBLE");
		else System.out.println(c/2); c%2!=0 c%2 c 2 0 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" System.out.println(c/2) System.out.println System.out System System.out c/2 c 2 String[] args args public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = Math.abs(b+a);
		if(c%2!=0) System.out.println("IMPOSSIBLE");
		else System.out.println(c/2);
	}
} public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = Math.abs(b+a);
		if(c%2!=0) System.out.println("IMPOSSIBLE");
		else System.out.println(c/2);
	}
} Main
/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = Math.abs(b+a);
		if(c%2!=0) System.out.println("IMPOSSIBLE");
		else System.out.println(c/2);
	}
}