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

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

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