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

public class Main
{
	public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int a=in.nextInt();
		int b=in.nextInt();
		if((a+b)%2!=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			System.out.println((a+b)/2);
		}
	}
}
	 import java.util.*; util.* java import java.io.*; io.* java public class Main
{
	public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int a=in.nextInt();
		int b=in.nextInt();
		if((a+b)%2!=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			System.out.println((a+b)/2);
		}
	}
} Main public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int a=in.nextInt();
		int b=in.nextInt();
		if((a+b)%2!=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			System.out.println((a+b)/2);
		}
	} main {
		Scanner in = new Scanner(System.in);
		int a=in.nextInt();
		int b=in.nextInt();
		if((a+b)%2!=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			System.out.println((a+b)/2);
		}
	} Scanner in = new Scanner(System.in); in new Scanner(System.in) int a=in.nextInt(); a in.nextInt() in.nextInt in int b=in.nextInt(); b in.nextInt() in.nextInt in if((a+b)%2!=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			System.out.println((a+b)/2);
		} (a+b)%2!=0 (a+b)%2 (a+b) a b 2 0 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			System.out.println((a+b)/2);
		} System.out.println((a+b)/2) System.out.println System.out System System.out (a+b)/2 (a+b) a b 2 String [] Args Args public class Main
{
	public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int a=in.nextInt();
		int b=in.nextInt();
		if((a+b)%2!=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			System.out.println((a+b)/2);
		}
	}
} public class Main
{
	public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int a=in.nextInt();
		int b=in.nextInt();
		if((a+b)%2!=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			System.out.println((a+b)/2);
		}
	}
} Main
import java.util.*;
import java.io.*;

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