
import java.util.Scanner;
import java.util.Arrays;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) {
        //コード
        Scanner scanner = new Scanner(System.in);
        long a = scanner.nextLong();
        long b = scanner.nextLong();
        if ((a+b) %2 == 0) {
            System.out.println((a+b) /2);
        } else {
            System.out.println("IMPOSSIBLE");
        }

    }
} import java.util.Scanner; Scanner java.util import java.util.Arrays; Arrays java.util import java.util.stream.*; stream.* java.util public class Main {
    public static void main(String[] args) {
        //コード
        Scanner scanner = new Scanner(System.in);
        long a = scanner.nextLong();
        long b = scanner.nextLong();
        if ((a+b) %2 == 0) {
            System.out.println((a+b) /2);
        } else {
            System.out.println("IMPOSSIBLE");
        }

    }
} Main public static void main(String[] args) {
        //コード
        Scanner scanner = new Scanner(System.in);
        long a = scanner.nextLong();
        long b = scanner.nextLong();
        if ((a+b) %2 == 0) {
            System.out.println((a+b) /2);
        } else {
            System.out.println("IMPOSSIBLE");
        }

    } main {
        //コード
        Scanner scanner = new Scanner(System.in);
        long a = scanner.nextLong();
        long b = scanner.nextLong();
        if ((a+b) %2 == 0) {
            System.out.println((a+b) /2);
        } else {
            System.out.println("IMPOSSIBLE");
        }

    } Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) long a = scanner.nextLong(); a scanner.nextLong() scanner.nextLong scanner long b = scanner.nextLong(); b scanner.nextLong() scanner.nextLong scanner 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) {
        //コード
        Scanner scanner = new Scanner(System.in);
        long a = scanner.nextLong();
        long b = scanner.nextLong();
        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) {
        //コード
        Scanner scanner = new Scanner(System.in);
        long a = scanner.nextLong();
        long b = scanner.nextLong();
        if ((a+b) %2 == 0) {
            System.out.println((a+b) /2);
        } else {
            System.out.println("IMPOSSIBLE");
        }

    }
} Main

import java.util.Scanner;
import java.util.Arrays;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) {
        //コード
        Scanner scanner = new Scanner(System.in);
        long a = scanner.nextLong();
        long b = scanner.nextLong();
        if ((a+b) %2 == 0) {
            System.out.println((a+b) /2);
        } else {
            System.out.println("IMPOSSIBLE");
        }

    }
}