import java.util.Scanner;
import java.util.HashMap;
import java.util.List;
import java.util.Arrays;
import java.util.ArrayList;
import java.lang.Math;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if ((a+b) % 2 == 0) {
            System.out.println((a+b) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }

} import java.util.Scanner; Scanner java.util import java.util.HashMap; HashMap java.util import java.util.List; List java.util import java.util.Arrays; Arrays java.util import java.util.ArrayList; ArrayList java.util import java.lang.Math; Math java.lang public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if ((a+b) % 2 == 0) {
            System.out.println((a+b) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }

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

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if ((a+b) % 2 == 0) {
            System.out.println((a+b) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }

} public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if ((a+b) % 2 == 0) {
            System.out.println((a+b) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }

} Main
import java.util.Scanner;
import java.util.HashMap;
import java.util.List;
import java.util.Arrays;
import java.util.ArrayList;
import java.lang.Math;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if ((a+b) % 2 == 0) {
            System.out.println((a+b) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }

}
