import java.util.*;
public class Main {
	public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String[] line = sc.nextLine().split(" ");
    int a = Integer.parseInt(line[0]);
    int b = Integer.parseInt(line[1]);
    if ((a + b) % 2 == 0) {
    	System.out.println((a + b) / 2);
    } else {
    	System.out.println("IMPOSSIBLE");
    }
    
    }
} import java.util.*; util.* java public class Main {
	public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String[] line = sc.nextLine().split(" ");
    int a = Integer.parseInt(line[0]);
    int b = Integer.parseInt(line[1]);
    if ((a + b) % 2 == 0) {
    	System.out.println((a + b) / 2);
    } else {
    	System.out.println("IMPOSSIBLE");
    }
    
    }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String[] line = sc.nextLine().split(" ");
    int a = Integer.parseInt(line[0]);
    int b = Integer.parseInt(line[1]);
    if ((a + b) % 2 == 0) {
    	System.out.println((a + b) / 2);
    } else {
    	System.out.println("IMPOSSIBLE");
    }
    
    } main {
    Scanner sc = new Scanner(System.in);
    String[] line = sc.nextLine().split(" ");
    int a = Integer.parseInt(line[0]);
    int b = Integer.parseInt(line[1]);
    if ((a + b) % 2 == 0) {
    	System.out.println((a + b) / 2);
    } else {
    	System.out.println("IMPOSSIBLE");
    }
    
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String[] line = sc.nextLine().split(" "); line sc.nextLine().split(" ") sc.nextLine().split sc.nextLine() sc.nextLine sc " " int a = Integer.parseInt(line[0]); a Integer.parseInt(line[0]) Integer.parseInt Integer line[0] line 0 int b = Integer.parseInt(line[1]); b Integer.parseInt(line[1]) Integer.parseInt Integer line[1] line 1 if ((a + b) % 2 == 0) {
    	System.out.println((a + b) / 2);
    } else {
    	System.out.println("IMPOSSIBLE");
    } (a + b) % 2 == 0 (a + b) % 2 (a + b) a b 2 0 {
    	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 {
    	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[] line = sc.nextLine().split(" ");
    int a = Integer.parseInt(line[0]);
    int b = Integer.parseInt(line[1]);
    if ((a + b) % 2 == 0) {
    	System.out.println((a + b) / 2);
    } else {
    	System.out.println("IMPOSSIBLE");
    }
    
    }
} public class Main {
	public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String[] line = sc.nextLine().split(" ");
    int a = Integer.parseInt(line[0]);
    int b = Integer.parseInt(line[1]);
    if ((a + b) % 2 == 0) {
    	System.out.println((a + b) / 2);
    } else {
    	System.out.println("IMPOSSIBLE");
    }
    
    }
} Main
import java.util.*;
public class Main {
	public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String[] line = sc.nextLine().split(" ");
    int a = Integer.parseInt(line[0]);
    int b = Integer.parseInt(line[1]);
    if ((a + b) % 2 == 0) {
    	System.out.println((a + b) / 2);
    } else {
    	System.out.println("IMPOSSIBLE");
    }
    
    }
}