import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        for (int k = 0; k <= 1000000000; k++) {
            if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            }
            if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            }
            if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            }
        }
        
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        for (int k = 0; k <= 1000000000; k++) {
            if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            }
            if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            }
            if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            }
        }
        
    }
} Main public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        for (int k = 0; k <= 1000000000; k++) {
            if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            }
            if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            }
            if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            }
        }
        
    } main {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        for (int k = 0; k <= 1000000000; k++) {
            if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            }
            if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            }
            if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            }
        }
        
    } 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 for (int k = 0; k <= 1000000000; k++) {
            if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            }
            if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            }
            if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            }
        } int k = 0; int k = 0; k 0 k <= 1000000000 k 1000000000 k++ k++ k {
            if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            }
            if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            }
            if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            }
        } {
            if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            }
            if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            }
            if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            }
        } if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            } Math.abs(A - k) == Math.abs(B - k) Math.abs(A - k) Math.abs Math A - k A k Math.abs(B - k) Math.abs Math B - k B k {
                System.out.println(k);
                break;
            } System.out.println(k) System.out.println System.out System System.out k break; if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            } A > B && k >= A A > B A B k >= A k A {
                System.out.println("IMPOSSIBLE");
                break;
            } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" break; if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            } B > A && k >= B B > A B A k >= B k B {
                System.out.println("IMPOSSIBLE");
                break;
            } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" break; String[] args args public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        for (int k = 0; k <= 1000000000; k++) {
            if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            }
            if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            }
            if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            }
        }
        
    }
} public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        for (int k = 0; k <= 1000000000; k++) {
            if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            }
            if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            }
            if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            }
        }
        
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        for (int k = 0; k <= 1000000000; k++) {
            if (Math.abs(A - k) == Math.abs(B - k)) {
                System.out.println(k);
                break;
            }
            if (A > B && k >= A) {
                System.out.println("IMPOSSIBLE");
                break;
            }
            if (B > A && k >= B) {
                System.out.println("IMPOSSIBLE");
                break;
            }
        }
        
    }
}
