/* 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. */
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();
    double c=(a+b)/2.0;
    if((int)c==c)
      System.out.println((int)c);
    else 
      System.out.println("IMPOSSIBLE");

	}
} import java.util.*; util.* java import java.lang.*; lang.* java import java.io.*; io.* java 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();
    double c=(a+b)/2.0;
    if((int)c==c)
      System.out.println((int)c);
    else 
      System.out.println("IMPOSSIBLE");

	}
} 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();
    double c=(a+b)/2.0;
    if((int)c==c)
      System.out.println((int)c);
    else 
      System.out.println("IMPOSSIBLE");

	} main {
		// your code goes here
		Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    double c=(a+b)/2.0;
    if((int)c==c)
      System.out.println((int)c);
    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 double c=(a+b)/2.0; c (a+b)/2.0 (a+b) a b 2.0 if((int)c==c)
      System.out.println((int)c);
    else 
      System.out.println("IMPOSSIBLE"); (int)c==c (int)c c System.out.println((int)c) System.out.println System.out System System.out (int)c System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args
/* 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. */
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();
    double c=(a+b)/2.0;
    if((int)c==c)
      System.out.println((int)c);
    else 
      System.out.println("IMPOSSIBLE");

	}
}