import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int first = scan.nextInt();
        int second = scan.nextInt();
        String result = "";
        int flg = (first + second) % 2;
        if (flg == 1) {
            result = "IMPOSSIBLE";
        } else {
            result = String.valueOf((first + second) / 2);
        }
        
        System.out.println(result);
    }
 } import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int first = scan.nextInt();
        int second = scan.nextInt();
        String result = "";
        int flg = (first + second) % 2;
        if (flg == 1) {
            result = "IMPOSSIBLE";
        } else {
            result = String.valueOf((first + second) / 2);
        }
        
        System.out.println(result);
    }
 } Main public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int first = scan.nextInt();
        int second = scan.nextInt();
        String result = "";
        int flg = (first + second) % 2;
        if (flg == 1) {
            result = "IMPOSSIBLE";
        } else {
            result = String.valueOf((first + second) / 2);
        }
        
        System.out.println(result);
    } main {
        Scanner scan = new Scanner(System.in);

        int first = scan.nextInt();
        int second = scan.nextInt();
        String result = "";
        int flg = (first + second) % 2;
        if (flg == 1) {
            result = "IMPOSSIBLE";
        } else {
            result = String.valueOf((first + second) / 2);
        }
        
        System.out.println(result);
    } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int first = scan.nextInt(); first scan.nextInt() scan.nextInt scan int second = scan.nextInt(); second scan.nextInt() scan.nextInt scan String result = ""; result "" int flg = (first + second) % 2; flg (first + second) % 2 (first + second) first second 2 if (flg == 1) {
            result = "IMPOSSIBLE";
        } else {
            result = String.valueOf((first + second) / 2);
        } flg == 1 flg 1 {
            result = "IMPOSSIBLE";
        } result = "IMPOSSIBLE" result "IMPOSSIBLE" {
            result = String.valueOf((first + second) / 2);
        } result = String.valueOf((first + second) / 2) result String.valueOf((first + second) / 2) String.valueOf String (first + second) / 2 (first + second) first second 2 System.out.println(result) System.out.println System.out System System.out result String[] args args public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int first = scan.nextInt();
        int second = scan.nextInt();
        String result = "";
        int flg = (first + second) % 2;
        if (flg == 1) {
            result = "IMPOSSIBLE";
        } else {
            result = String.valueOf((first + second) / 2);
        }
        
        System.out.println(result);
    }
 } public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int first = scan.nextInt();
        int second = scan.nextInt();
        String result = "";
        int flg = (first + second) % 2;
        if (flg == 1) {
            result = "IMPOSSIBLE";
        } else {
            result = String.valueOf((first + second) / 2);
        }
        
        System.out.println(result);
    }
 } Main
import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int first = scan.nextInt();
        int second = scan.nextInt();
        String result = "";
        int flg = (first + second) % 2;
        if (flg == 1) {
            result = "IMPOSSIBLE";
        } else {
            result = String.valueOf((first + second) / 2);
        }
        
        System.out.println(result);
    }
 }