/*
 * @Author: Kaustav Vats  
 */
import java.io.*;
import java.util.*;
import java.math.*;
import java.lang.*;


class Main {

    public static void PrintIntArray(int[] Arr) {
        for (int i=0; i<Arr.length; i++) {
            System.out.print(Arr[i] + " ");
        }
        System.out.println();
    }

    public static void main(String[] args) throws IOException{
        Reader.init(System.in);
        long A = Reader.nextLong();
        long B = Reader.nextLong();
        if ((A+B) % 2 == 0) {
            System.out.println((A + B)/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
    }
}


/** Class for buffered reading int and double values */
class Reader {
    static BufferedReader reader;
    static StringTokenizer tokenizer;

    /** call this method to initialize reader for InputStream */
    static void init(InputStream input) {
        reader = new BufferedReader(
                     new InputStreamReader(input) );
        tokenizer = new StringTokenizer("");
    }

    /** get next word */
    static String next() throws IOException {
        while ( ! tokenizer.hasMoreTokens() ) {
            //TODO add check for eof if necessary
            tokenizer = new StringTokenizer(
                   reader.readLine() );
        }
        return tokenizer.nextToken();
    }

    static int nextInt() throws IOException {
        return Integer.parseInt( next() );
    }

    static long nextLong() throws IOException {
        return Long.parseLong( next() );
    }
	
    static double nextDouble() throws IOException {
        return Double.parseDouble( next() );
    }
} import java.io.*; io.* java import java.util.*; util.* java import java.math.*; math.* java import java.lang.*; lang.* java class Main {

    public static void PrintIntArray(int[] Arr) {
        for (int i=0; i<Arr.length; i++) {
            System.out.print(Arr[i] + " ");
        }
        System.out.println();
    }

    public static void main(String[] args) throws IOException{
        Reader.init(System.in);
        long A = Reader.nextLong();
        long B = Reader.nextLong();
        if ((A+B) % 2 == 0) {
            System.out.println((A + B)/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
    }
} Main public static void PrintIntArray(int[] Arr) {
        for (int i=0; i<Arr.length; i++) {
            System.out.print(Arr[i] + " ");
        }
        System.out.println();
    } PrintIntArray {
        for (int i=0; i<Arr.length; i++) {
            System.out.print(Arr[i] + " ");
        }
        System.out.println();
    } for (int i=0; i<Arr.length; i++) {
            System.out.print(Arr[i] + " ");
        } int i=0; int i=0; i 0 i<Arr.length i Arr.length Arr Arr.length i++ i++ i {
            System.out.print(Arr[i] + " ");
        } {
            System.out.print(Arr[i] + " ");
        } System.out.print(Arr[i] + " ") System.out.print System.out System System.out Arr[i] + " " Arr[i] Arr i " " System.out.println() System.out.println System.out System System.out int[] Arr Arr public static void main(String[] args) throws IOException{
        Reader.init(System.in);
        long A = Reader.nextLong();
        long B = Reader.nextLong();
        if ((A+B) % 2 == 0) {
            System.out.println((A + B)/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
    } main {
        Reader.init(System.in);
        long A = Reader.nextLong();
        long B = Reader.nextLong();
        if ((A+B) % 2 == 0) {
            System.out.println((A + B)/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
    } Reader.init(System.in) Reader.init Reader System.in System System.in long A = Reader.nextLong(); A Reader.nextLong() Reader.nextLong Reader long B = Reader.nextLong(); B Reader.nextLong() Reader.nextLong Reader 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 class Reader {
    static BufferedReader reader;
    static StringTokenizer tokenizer;

    /** call this method to initialize reader for InputStream */
    static void init(InputStream input) {
        reader = new BufferedReader(
                     new InputStreamReader(input) );
        tokenizer = new StringTokenizer("");
    }

    /** get next word */
    static String next() throws IOException {
        while ( ! tokenizer.hasMoreTokens() ) {
            //TODO add check for eof if necessary
            tokenizer = new StringTokenizer(
                   reader.readLine() );
        }
        return tokenizer.nextToken();
    }

    static int nextInt() throws IOException {
        return Integer.parseInt( next() );
    }

    static long nextLong() throws IOException {
        return Long.parseLong( next() );
    }
	
    static double nextDouble() throws IOException {
        return Double.parseDouble( next() );
    }
} Reader static BufferedReader reader; reader static StringTokenizer tokenizer; tokenizer /** call this method to initialize reader for InputStream */
    static void init(InputStream input) {
        reader = new BufferedReader(
                     new InputStreamReader(input) );
        tokenizer = new StringTokenizer("");
    } init {
        reader = new BufferedReader(
                     new InputStreamReader(input) );
        tokenizer = new StringTokenizer("");
    } reader = new BufferedReader(
                     new InputStreamReader(input) ) reader new BufferedReader(
                     new InputStreamReader(input) ) tokenizer = new StringTokenizer("") tokenizer new StringTokenizer("") InputStream input input /** get next word */
    static String next() throws IOException {
        while ( ! tokenizer.hasMoreTokens() ) {
            //TODO add check for eof if necessary
            tokenizer = new StringTokenizer(
                   reader.readLine() );
        }
        return tokenizer.nextToken();
    } next {
        while ( ! tokenizer.hasMoreTokens() ) {
            //TODO add check for eof if necessary
            tokenizer = new StringTokenizer(
                   reader.readLine() );
        }
        return tokenizer.nextToken();
    } while ( ! tokenizer.hasMoreTokens() ) {
            //TODO add check for eof if necessary
            tokenizer = new StringTokenizer(
                   reader.readLine() );
        } ! tokenizer.hasMoreTokens() tokenizer.hasMoreTokens() tokenizer.hasMoreTokens tokenizer {
            //TODO add check for eof if necessary
            tokenizer = new StringTokenizer(
                   reader.readLine() );
        } tokenizer = new StringTokenizer(
                   reader.readLine() ) tokenizer new StringTokenizer(
                   reader.readLine() ) return tokenizer.nextToken(); tokenizer.nextToken() tokenizer.nextToken tokenizer static int nextInt() throws IOException {
        return Integer.parseInt( next() );
    } nextInt {
        return Integer.parseInt( next() );
    } return Integer.parseInt( next() ); Integer.parseInt( next() ) Integer.parseInt Integer next() next static long nextLong() throws IOException {
        return Long.parseLong( next() );
    } nextLong {
        return Long.parseLong( next() );
    } return Long.parseLong( next() ); Long.parseLong( next() ) Long.parseLong Long next() next static double nextDouble() throws IOException {
        return Double.parseDouble( next() );
    } nextDouble {
        return Double.parseDouble( next() );
    } return Double.parseDouble( next() ); Double.parseDouble( next() ) Double.parseDouble Double next() next
/*
 * @Author: Kaustav Vats  
 */
import java.io.*;
import java.util.*;
import java.math.*;
import java.lang.*;


class Main {

    public static void PrintIntArray(int[] Arr) {
        for (int i=0; i<Arr.length; i++) {
            System.out.print(Arr[i] + " ");
        }
        System.out.println();
    }

    public static void main(String[] args) throws IOException{
        Reader.init(System.in);
        long A = Reader.nextLong();
        long B = Reader.nextLong();
        if ((A+B) % 2 == 0) {
            System.out.println((A + B)/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
    }
}


/** Class for buffered reading int and double values */
class Reader {
    static BufferedReader reader;
    static StringTokenizer tokenizer;

    /** call this method to initialize reader for InputStream */
    static void init(InputStream input) {
        reader = new BufferedReader(
                     new InputStreamReader(input) );
        tokenizer = new StringTokenizer("");
    }

    /** get next word */
    static String next() throws IOException {
        while ( ! tokenizer.hasMoreTokens() ) {
            //TODO add check for eof if necessary
            tokenizer = new StringTokenizer(
                   reader.readLine() );
        }
        return tokenizer.nextToken();
    }

    static int nextInt() throws IOException {
        return Integer.parseInt( next() );
    }

    static long nextLong() throws IOException {
        return Long.parseLong( next() );
    }
	
    static double nextDouble() throws IOException {
        return Double.parseDouble( next() );
    }
}