import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {

    public static void main(String[] args) throws Exception {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            String[] array = reader.readLine().split(" ");
            int A = Integer.parseInt(array[0]);
            int B = Integer.parseInt(array[1]);

            if ((A + B) % 2 == 1) {
                System.out.println("IMPOSSIBLE");
            } else {
                System.out.println((A + B) / 2);
            }
        }
    }

} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io public class Main {

    public static void main(String[] args) throws Exception {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            String[] array = reader.readLine().split(" ");
            int A = Integer.parseInt(array[0]);
            int B = Integer.parseInt(array[1]);

            if ((A + B) % 2 == 1) {
                System.out.println("IMPOSSIBLE");
            } else {
                System.out.println((A + B) / 2);
            }
        }
    }

} Main public static void main(String[] args) throws Exception {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            String[] array = reader.readLine().split(" ");
            int A = Integer.parseInt(array[0]);
            int B = Integer.parseInt(array[1]);

            if ((A + B) % 2 == 1) {
                System.out.println("IMPOSSIBLE");
            } else {
                System.out.println((A + B) / 2);
            }
        }
    } main {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            String[] array = reader.readLine().split(" ");
            int A = Integer.parseInt(array[0]);
            int B = Integer.parseInt(array[1]);

            if ((A + B) % 2 == 1) {
                System.out.println("IMPOSSIBLE");
            } else {
                System.out.println((A + B) / 2);
            }
        }
    } try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            String[] array = reader.readLine().split(" ");
            int A = Integer.parseInt(array[0]);
            int B = Integer.parseInt(array[1]);

            if ((A + B) % 2 == 1) {
                System.out.println("IMPOSSIBLE");
            } else {
                System.out.println((A + B) / 2);
            }
        } {
            String[] array = reader.readLine().split(" ");
            int A = Integer.parseInt(array[0]);
            int B = Integer.parseInt(array[1]);

            if ((A + B) % 2 == 1) {
                System.out.println("IMPOSSIBLE");
            } else {
                System.out.println((A + B) / 2);
            }
        } BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)) BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)) new BufferedReader(new InputStreamReader(System.in)) String[] array = reader.readLine().split(" "); array reader.readLine().split(" ") reader.readLine().split reader.readLine() reader.readLine reader " " int A = Integer.parseInt(array[0]); A Integer.parseInt(array[0]) Integer.parseInt Integer array[0] array 0 int B = Integer.parseInt(array[1]); B Integer.parseInt(array[1]) Integer.parseInt Integer array[1] array 1 if ((A + B) % 2 == 1) {
                System.out.println("IMPOSSIBLE");
            } else {
                System.out.println((A + B) / 2);
            } (A + B) % 2 == 1 (A + B) % 2 (A + B) A B 2 1 {
                System.out.println("IMPOSSIBLE");
            } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
                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 String[] args args public class Main {

    public static void main(String[] args) throws Exception {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            String[] array = reader.readLine().split(" ");
            int A = Integer.parseInt(array[0]);
            int B = Integer.parseInt(array[1]);

            if ((A + B) % 2 == 1) {
                System.out.println("IMPOSSIBLE");
            } else {
                System.out.println((A + B) / 2);
            }
        }
    }

} public class Main {

    public static void main(String[] args) throws Exception {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            String[] array = reader.readLine().split(" ");
            int A = Integer.parseInt(array[0]);
            int B = Integer.parseInt(array[1]);

            if ((A + B) % 2 == 1) {
                System.out.println("IMPOSSIBLE");
            } else {
                System.out.println((A + B) / 2);
            }
        }
    }

} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {

    public static void main(String[] args) throws Exception {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            String[] array = reader.readLine().split(" ");
            int A = Integer.parseInt(array[0]);
            int B = Integer.parseInt(array[1]);

            if ((A + B) % 2 == 1) {
                System.out.println("IMPOSSIBLE");
            } else {
                System.out.println((A + B) / 2);
            }
        }
    }

}
