import java.util.*;

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

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