import java.util.Scanner;

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){
            int tmp = A;
            A=B;
            B=tmp;
        }
        double ans = (A+B)/2.0;
        int floor = (int)ans;
        double finalnum = ans-floor;
        if (finalnum==0){
            System.out.println(floor);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    }
} import java.util.Scanner; Scanner java.util 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){
            int tmp = A;
            A=B;
            B=tmp;
        }
        double ans = (A+B)/2.0;
        int floor = (int)ans;
        double finalnum = ans-floor;
        if (finalnum==0){
            System.out.println(floor);
        }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){
            int tmp = A;
            A=B;
            B=tmp;
        }
        double ans = (A+B)/2.0;
        int floor = (int)ans;
        double finalnum = ans-floor;
        if (finalnum==0){
            System.out.println(floor);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    } main {

        Scanner sc = new Scanner(System.in);
        int A=sc.nextInt();
        int B = sc.nextInt();
        if (A<B){
            int tmp = A;
            A=B;
            B=tmp;
        }
        double ans = (A+B)/2.0;
        int floor = (int)ans;
        double finalnum = ans-floor;
        if (finalnum==0){
            System.out.println(floor);
        }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){
            int tmp = A;
            A=B;
            B=tmp;
        } A<B A B {
            int tmp = A;
            A=B;
            B=tmp;
        } int tmp = A; tmp A A=B A B B=tmp B tmp double ans = (A+B)/2.0; ans (A+B)/2.0 (A+B) A B 2.0 int floor = (int)ans; floor (int)ans double finalnum = ans-floor; finalnum ans-floor ans floor if (finalnum==0){
            System.out.println(floor);
        }else{
            System.out.println("IMPOSSIBLE");
        } finalnum==0 finalnum 0 {
            System.out.println(floor);
        } System.out.println(floor) System.out.println System.out System System.out floor {
            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){
            int tmp = A;
            A=B;
            B=tmp;
        }
        double ans = (A+B)/2.0;
        int floor = (int)ans;
        double finalnum = ans-floor;
        if (finalnum==0){
            System.out.println(floor);
        }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){
            int tmp = A;
            A=B;
            B=tmp;
        }
        double ans = (A+B)/2.0;
        int floor = (int)ans;
        double finalnum = ans-floor;
        if (finalnum==0){
            System.out.println(floor);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    }
} Main
import java.util.Scanner;

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){
            int tmp = A;
            A=B;
            B=tmp;
        }
        double ans = (A+B)/2.0;
        int floor = (int)ans;
        double finalnum = ans-floor;
        if (finalnum==0){
            System.out.println(floor);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    }
}
