import java.util.*;
public class Main {
    public static void main(String[] args){

        Scanner sc = new Scanner(System.in);
        // 整数の入力
        long A=sc.nextInt();
        long B=sc.nextInt();
        if((A+B)%2==0){
            System.out.println((A+B)/2);
        }else{
            System.out.println("IMPOSSIBLE");

        }







    }
    static int gcd (int a, int b) {
        int temp;
        while((temp = a%b)!=0) {
            a = b;
            b = temp;
        }
        return b;
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args){

        Scanner sc = new Scanner(System.in);
        // 整数の入力
        long A=sc.nextInt();
        long B=sc.nextInt();
        if((A+B)%2==0){
            System.out.println((A+B)/2);
        }else{
            System.out.println("IMPOSSIBLE");

        }







    }
    static int gcd (int a, int b) {
        int temp;
        while((temp = a%b)!=0) {
            a = b;
            b = temp;
        }
        return b;
    }
} Main public static void main(String[] args){

        Scanner sc = new Scanner(System.in);
        // 整数の入力
        long A=sc.nextInt();
        long B=sc.nextInt();
        if((A+B)%2==0){
            System.out.println((A+B)/2);
        }else{
            System.out.println("IMPOSSIBLE");

        }







    } main {

        Scanner sc = new Scanner(System.in);
        // 整数の入力
        long A=sc.nextInt();
        long B=sc.nextInt();
        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) long A=sc.nextInt(); A sc.nextInt() sc.nextInt sc long B=sc.nextInt(); B sc.nextInt() sc.nextInt sc 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 static int gcd (int a, int b) {
        int temp;
        while((temp = a%b)!=0) {
            a = b;
            b = temp;
        }
        return b;
    } gcd {
        int temp;
        while((temp = a%b)!=0) {
            a = b;
            b = temp;
        }
        return b;
    } int temp; temp while((temp = a%b)!=0) {
            a = b;
            b = temp;
        } (temp = a%b)!=0 (temp = a%b) temp a%b a b 0 {
            a = b;
            b = temp;
        } a = b a b b = temp b temp return b; b int a a int b b public class Main {
    public static void main(String[] args){

        Scanner sc = new Scanner(System.in);
        // 整数の入力
        long A=sc.nextInt();
        long B=sc.nextInt();
        if((A+B)%2==0){
            System.out.println((A+B)/2);
        }else{
            System.out.println("IMPOSSIBLE");

        }







    }
    static int gcd (int a, int b) {
        int temp;
        while((temp = a%b)!=0) {
            a = b;
            b = temp;
        }
        return b;
    }
} public class Main {
    public static void main(String[] args){

        Scanner sc = new Scanner(System.in);
        // 整数の入力
        long A=sc.nextInt();
        long B=sc.nextInt();
        if((A+B)%2==0){
            System.out.println((A+B)/2);
        }else{
            System.out.println("IMPOSSIBLE");

        }







    }
    static int gcd (int a, int b) {
        int temp;
        while((temp = a%b)!=0) {
            a = b;
            b = temp;
        }
        return b;
    }
} Main
import java.util.*;
public class Main {
    public static void main(String[] args){

        Scanner sc = new Scanner(System.in);
        // 整数の入力
        long A=sc.nextInt();
        long B=sc.nextInt();
        if((A+B)%2==0){
            System.out.println((A+B)/2);
        }else{
            System.out.println("IMPOSSIBLE");

        }







    }
    static int gcd (int a, int b) {
        int temp;
        while((temp = a%b)!=0) {
            a = b;
            b = temp;
        }
        return b;
    }
}
