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

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();

        int C = 0;
        int D = 0;

        C = (A+B)/2;
        D = (A+B)%2;

        if(D != 0){
            System.out.println("IMPOSSIBLE");
        }else{
            System.out.println(C);
        }


        

    }
} import java.util.*; util.* java import java.lang.*; lang.* java import java.io.*; io.* java public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();

        int C = 0;
        int D = 0;

        C = (A+B)/2;
        D = (A+B)%2;

        if(D != 0){
            System.out.println("IMPOSSIBLE");
        }else{
            System.out.println(C);
        }


        

    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();

        int C = 0;
        int D = 0;

        C = (A+B)/2;
        D = (A+B)%2;

        if(D != 0){
            System.out.println("IMPOSSIBLE");
        }else{
            System.out.println(C);
        }


        

    } main {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();

        int C = 0;
        int D = 0;

        C = (A+B)/2;
        D = (A+B)%2;

        if(D != 0){
            System.out.println("IMPOSSIBLE");
        }else{
            System.out.println(C);
        }


        

    } 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 int C = 0; C 0 int D = 0; D 0 C = (A+B)/2 C (A+B)/2 (A+B) A B 2 D = (A+B)%2 D (A+B)%2 (A+B) A B 2 if(D != 0){
            System.out.println("IMPOSSIBLE");
        }else{
            System.out.println(C);
        } D != 0 D 0 {
            System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
            System.out.println(C);
        } System.out.println(C) System.out.println System.out System System.out C String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();

        int C = 0;
        int D = 0;

        C = (A+B)/2;
        D = (A+B)%2;

        if(D != 0){
            System.out.println("IMPOSSIBLE");
        }else{
            System.out.println(C);
        }


        

    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();

        int C = 0;
        int D = 0;

        C = (A+B)/2;
        D = (A+B)%2;

        if(D != 0){
            System.out.println("IMPOSSIBLE");
        }else{
            System.out.println(C);
        }


        

    }
} Main
import java.util.*;
import java.lang.*;
import java.io.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();

        int C = 0;
        int D = 0;

        C = (A+B)/2;
        D = (A+B)%2;

        if(D != 0){
            System.out.println("IMPOSSIBLE");
        }else{
            System.out.println(C);
        }


        

    }
}