import 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();
        sc.close();
        if ((A - B) % 2 == 0) {
            System.out.println((A + B) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        sc.close();
        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();
        sc.close();
        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();
        sc.close();
        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 sc.close() sc.close 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();
        sc.close();
        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();
        sc.close();
        if ((A - B) % 2 == 0) {
            System.out.println((A + B) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} Main
import 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();
        sc.close();
        if ((A - B) % 2 == 0) {
            System.out.println((A + B) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
}
