import java.util.Scanner;
import java.util.*;
 class Main{
public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int num1 = sc.nextInt();
        int num2 = sc.nextInt();
if(Math.abs(num1 + num2) % 2 ==0 || num1 - num2 ==0 ){
        System.out.println(Math.abs(num1 + num2) / 2);
}else{
  System.out.println("IMPOSSIBLE");
}
}
}
 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 num1 = sc.nextInt();
        int num2 = sc.nextInt();
if(Math.abs(num1 + num2) % 2 ==0 || num1 - num2 ==0 ){
        System.out.println(Math.abs(num1 + num2) / 2);
}else{
  System.out.println("IMPOSSIBLE");
}
}
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int num1 = sc.nextInt();
        int num2 = sc.nextInt();
if(Math.abs(num1 + num2) % 2 ==0 || num1 - num2 ==0 ){
        System.out.println(Math.abs(num1 + num2) / 2);
}else{
  System.out.println("IMPOSSIBLE");
}
} main {
        Scanner sc = new Scanner(System.in);
        int num1 = sc.nextInt();
        int num2 = sc.nextInt();
if(Math.abs(num1 + num2) % 2 ==0 || num1 - num2 ==0 ){
        System.out.println(Math.abs(num1 + num2) / 2);
}else{
  System.out.println("IMPOSSIBLE");
}
} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int num1 = sc.nextInt(); num1 sc.nextInt() sc.nextInt sc int num2 = sc.nextInt(); num2 sc.nextInt() sc.nextInt sc if(Math.abs(num1 + num2) % 2 ==0 || num1 - num2 ==0 ){
        System.out.println(Math.abs(num1 + num2) / 2);
}else{
  System.out.println("IMPOSSIBLE");
} Math.abs(num1 + num2) % 2 ==0 || num1 - num2 ==0 Math.abs(num1 + num2) % 2 ==0 Math.abs(num1 + num2) % 2 Math.abs(num1 + num2) Math.abs Math num1 + num2 num1 num2 2 0 num1 - num2 ==0 num1 - num2 num1 num2 0 {
        System.out.println(Math.abs(num1 + num2) / 2);
} System.out.println(Math.abs(num1 + num2) / 2) System.out.println System.out System System.out Math.abs(num1 + num2) / 2 Math.abs(num1 + num2) Math.abs Math num1 + num2 num1 num2 2 {
  System.out.println("IMPOSSIBLE");
} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" 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 num1 = sc.nextInt();
        int num2 = sc.nextInt();
if(Math.abs(num1 + num2) % 2 ==0 || num1 - num2 ==0 ){
        System.out.println(Math.abs(num1 + num2) / 2);
}else{
  System.out.println("IMPOSSIBLE");
}
}
}

