import java.util.Scanner;

public class Main {
    public static void debug(String str) {
        boolean debug = false;
        if (debug != false) {
            return;
        }
        System.out.println(str);
    }

    public static void main(String[] args) {

        long A;
        long B;

        try (Scanner sc = new Scanner(System.in)) {
            A = sc.nextLong();
            B = sc.nextLong();
        }
        long K = 0;
        if(Math.abs(A - B ) %2 ==1 ){
            System.out.println("IMPOSSIBLE");
            return;
        }
        if(A > B){
            K = A - ((A - B) / 2);
        }else{
            K = B - ((B - A) /2);
        }
        System.out.println(K);
        return;
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void debug(String str) {
        boolean debug = false;
        if (debug != false) {
            return;
        }
        System.out.println(str);
    }

    public static void main(String[] args) {

        long A;
        long B;

        try (Scanner sc = new Scanner(System.in)) {
            A = sc.nextLong();
            B = sc.nextLong();
        }
        long K = 0;
        if(Math.abs(A - B ) %2 ==1 ){
            System.out.println("IMPOSSIBLE");
            return;
        }
        if(A > B){
            K = A - ((A - B) / 2);
        }else{
            K = B - ((B - A) /2);
        }
        System.out.println(K);
        return;
    }
} Main public static void debug(String str) {
        boolean debug = false;
        if (debug != false) {
            return;
        }
        System.out.println(str);
    } debug {
        boolean debug = false;
        if (debug != false) {
            return;
        }
        System.out.println(str);
    } boolean debug = false; debug false if (debug != false) {
            return;
        } debug != false debug false {
            return;
        } return; System.out.println(str) System.out.println System.out System System.out str String str str public static void main(String[] args) {

        long A;
        long B;

        try (Scanner sc = new Scanner(System.in)) {
            A = sc.nextLong();
            B = sc.nextLong();
        }
        long K = 0;
        if(Math.abs(A - B ) %2 ==1 ){
            System.out.println("IMPOSSIBLE");
            return;
        }
        if(A > B){
            K = A - ((A - B) / 2);
        }else{
            K = B - ((B - A) /2);
        }
        System.out.println(K);
        return;
    } main {

        long A;
        long B;

        try (Scanner sc = new Scanner(System.in)) {
            A = sc.nextLong();
            B = sc.nextLong();
        }
        long K = 0;
        if(Math.abs(A - B ) %2 ==1 ){
            System.out.println("IMPOSSIBLE");
            return;
        }
        if(A > B){
            K = A - ((A - B) / 2);
        }else{
            K = B - ((B - A) /2);
        }
        System.out.println(K);
        return;
    } long A; A long B; B try (Scanner sc = new Scanner(System.in)) {
            A = sc.nextLong();
            B = sc.nextLong();
        } {
            A = sc.nextLong();
            B = sc.nextLong();
        } Scanner sc = new Scanner(System.in) Scanner sc = new Scanner(System.in) new Scanner(System.in) A = sc.nextLong() A sc.nextLong() sc.nextLong sc B = sc.nextLong() B sc.nextLong() sc.nextLong sc long K = 0; K 0 if(Math.abs(A - B ) %2 ==1 ){
            System.out.println("IMPOSSIBLE");
            return;
        } Math.abs(A - B ) %2 ==1 Math.abs(A - B ) %2 Math.abs(A - B ) Math.abs Math A - B A B 2 1 {
            System.out.println("IMPOSSIBLE");
            return;
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" return; if(A > B){
            K = A - ((A - B) / 2);
        }else{
            K = B - ((B - A) /2);
        } A > B A B {
            K = A - ((A - B) / 2);
        } K = A - ((A - B) / 2) K A - ((A - B) / 2) A ((A - B) / 2) (A - B) A B 2 {
            K = B - ((B - A) /2);
        } K = B - ((B - A) /2) K B - ((B - A) /2) B ((B - A) /2) (B - A) B A 2 System.out.println(K) System.out.println System.out System System.out K return; String[] args args public class Main {
    public static void debug(String str) {
        boolean debug = false;
        if (debug != false) {
            return;
        }
        System.out.println(str);
    }

    public static void main(String[] args) {

        long A;
        long B;

        try (Scanner sc = new Scanner(System.in)) {
            A = sc.nextLong();
            B = sc.nextLong();
        }
        long K = 0;
        if(Math.abs(A - B ) %2 ==1 ){
            System.out.println("IMPOSSIBLE");
            return;
        }
        if(A > B){
            K = A - ((A - B) / 2);
        }else{
            K = B - ((B - A) /2);
        }
        System.out.println(K);
        return;
    }
} public class Main {
    public static void debug(String str) {
        boolean debug = false;
        if (debug != false) {
            return;
        }
        System.out.println(str);
    }

    public static void main(String[] args) {

        long A;
        long B;

        try (Scanner sc = new Scanner(System.in)) {
            A = sc.nextLong();
            B = sc.nextLong();
        }
        long K = 0;
        if(Math.abs(A - B ) %2 ==1 ){
            System.out.println("IMPOSSIBLE");
            return;
        }
        if(A > B){
            K = A - ((A - B) / 2);
        }else{
            K = B - ((B - A) /2);
        }
        System.out.println(K);
        return;
    }
} Main
import java.util.Scanner;

public class Main {
    public static void debug(String str) {
        boolean debug = false;
        if (debug != false) {
            return;
        }
        System.out.println(str);
    }

    public static void main(String[] args) {

        long A;
        long B;

        try (Scanner sc = new Scanner(System.in)) {
            A = sc.nextLong();
            B = sc.nextLong();
        }
        long K = 0;
        if(Math.abs(A - B ) %2 ==1 ){
            System.out.println("IMPOSSIBLE");
            return;
        }
        if(A > B){
            K = A - ((A - B) / 2);
        }else{
            K = B - ((B - A) /2);
        }
        System.out.println(K);
        return;
    }
}