

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

        if (add%2 == 1){
            System.out.println("IMPOSSIBLE");
        }
        else{
            System.out.println(add/2);
        }
    }
} import java.util.Scanner; Scanner java.util import java.util.*; util.* java class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int add  = (A+B);

        if (add%2 == 1){
            System.out.println("IMPOSSIBLE");
        }
        else{
            System.out.println(add/2);
        }
    }
} Main public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int add  = (A+B);

        if (add%2 == 1){
            System.out.println("IMPOSSIBLE");
        }
        else{
            System.out.println(add/2);
        }
    } main {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int add  = (A+B);

        if (add%2 == 1){
            System.out.println("IMPOSSIBLE");
        }
        else{
            System.out.println(add/2);
        }
    } 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 add  = (A+B); add (A+B) A B if (add%2 == 1){
            System.out.println("IMPOSSIBLE");
        }
        else{
            System.out.println(add/2);
        } add%2 == 1 add%2 add 2 1 {
            System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
            System.out.println(add/2);
        } System.out.println(add/2) System.out.println System.out System System.out add/2 add 2 String[] args args


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

        if (add%2 == 1){
            System.out.println("IMPOSSIBLE");
        }
        else{
            System.out.println(add/2);
        }
    }
}
