import java.util.*;
import java.io.*;

public class Main{

    public static void main(String[] args) throws Exception{

        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);

        Task solver = new Task();
        solver.solve(in,out);
        out.close();
    }

    static class Task{

        public void solve(InputReader in ,PrintWriter out){

            long a = in.nextLong();
            long b = in.nextLong();
            long mid = (a+b)/2;
            if(Math.abs(a - mid) == Math.abs(b - mid))
              out.println(mid);
            else 
              out.println("IMPOSSIBLE");
        }
    }

    static class InputReader{

        BufferedReader reader;
        StringTokenizer tokenizer;

        public InputReader(InputStream inputStream){
            reader = new BufferedReader(new InputStreamReader(inputStream));
            tokenizer = null;
        }

        public String next(){
            while(tokenizer == null || !tokenizer.hasMoreTokens()){
                try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public long nextLong(){
            return Long.parseLong(next());
        }
    }
} import java.util.*; util.* java import java.io.*; io.* java public class Main{

    public static void main(String[] args) throws Exception{

        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);

        Task solver = new Task();
        solver.solve(in,out);
        out.close();
    }

    static class Task{

        public void solve(InputReader in ,PrintWriter out){

            long a = in.nextLong();
            long b = in.nextLong();
            long mid = (a+b)/2;
            if(Math.abs(a - mid) == Math.abs(b - mid))
              out.println(mid);
            else 
              out.println("IMPOSSIBLE");
        }
    }

    static class InputReader{

        BufferedReader reader;
        StringTokenizer tokenizer;

        public InputReader(InputStream inputStream){
            reader = new BufferedReader(new InputStreamReader(inputStream));
            tokenizer = null;
        }

        public String next(){
            while(tokenizer == null || !tokenizer.hasMoreTokens()){
                try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public long nextLong(){
            return Long.parseLong(next());
        }
    }
} Main public static void main(String[] args) throws Exception{

        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);

        Task solver = new Task();
        solver.solve(in,out);
        out.close();
    } main {

        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);

        Task solver = new Task();
        solver.solve(in,out);
        out.close();
    } InputStream inputStream = System.in; inputStream System.in System System.in OutputStream outputStream = System.out; outputStream System.out System System.out InputReader in = new InputReader(inputStream); in new InputReader(inputStream) PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) Task solver = new Task(); solver new Task() solver.solve(in,out) solver.solve solver in out out.close() out.close out String[] args args static class Task{

        public void solve(InputReader in ,PrintWriter out){

            long a = in.nextLong();
            long b = in.nextLong();
            long mid = (a+b)/2;
            if(Math.abs(a - mid) == Math.abs(b - mid))
              out.println(mid);
            else 
              out.println("IMPOSSIBLE");
        }
    } Task public void solve(InputReader in ,PrintWriter out){

            long a = in.nextLong();
            long b = in.nextLong();
            long mid = (a+b)/2;
            if(Math.abs(a - mid) == Math.abs(b - mid))
              out.println(mid);
            else 
              out.println("IMPOSSIBLE");
        } solve {

            long a = in.nextLong();
            long b = in.nextLong();
            long mid = (a+b)/2;
            if(Math.abs(a - mid) == Math.abs(b - mid))
              out.println(mid);
            else 
              out.println("IMPOSSIBLE");
        } long a = in.nextLong(); a in.nextLong() in.nextLong in long b = in.nextLong(); b in.nextLong() in.nextLong in long mid = (a+b)/2; mid (a+b)/2 (a+b) a b 2 if(Math.abs(a - mid) == Math.abs(b - mid))
              out.println(mid);
            else 
              out.println("IMPOSSIBLE"); Math.abs(a - mid) == Math.abs(b - mid) Math.abs(a - mid) Math.abs Math a - mid a mid Math.abs(b - mid) Math.abs Math b - mid b mid out.println(mid) out.println out mid out.println("IMPOSSIBLE") out.println out "IMPOSSIBLE" InputReader in in PrintWriter out out static class InputReader{

        BufferedReader reader;
        StringTokenizer tokenizer;

        public InputReader(InputStream inputStream){
            reader = new BufferedReader(new InputStreamReader(inputStream));
            tokenizer = null;
        }

        public String next(){
            while(tokenizer == null || !tokenizer.hasMoreTokens()){
                try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public long nextLong(){
            return Long.parseLong(next());
        }
    } InputReader BufferedReader reader; reader StringTokenizer tokenizer; tokenizer public InputReader(InputStream inputStream){
            reader = new BufferedReader(new InputStreamReader(inputStream));
            tokenizer = null;
        } InputReader {
            reader = new BufferedReader(new InputStreamReader(inputStream));
            tokenizer = null;
        } reader = new BufferedReader(new InputStreamReader(inputStream)) reader new BufferedReader(new InputStreamReader(inputStream)) tokenizer = null tokenizer null InputStream inputStream inputStream public String next(){
            while(tokenizer == null || !tokenizer.hasMoreTokens()){
                try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        } next {
            while(tokenizer == null || !tokenizer.hasMoreTokens()){
                try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        } while(tokenizer == null || !tokenizer.hasMoreTokens()){
                try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                }
            } tokenizer == null || !tokenizer.hasMoreTokens() tokenizer == null tokenizer null !tokenizer.hasMoreTokens() tokenizer.hasMoreTokens() tokenizer.hasMoreTokens tokenizer {
                try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                }
            } try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                } catch(IOException e){
                    throw new RuntimeException(e);
                } IOException e {
                    throw new RuntimeException(e);
                } throw new RuntimeException(e); new RuntimeException(e) {
                    tokenizer = new StringTokenizer(reader.readLine());
                } tokenizer = new StringTokenizer(reader.readLine()) tokenizer new StringTokenizer(reader.readLine()) return tokenizer.nextToken(); tokenizer.nextToken() tokenizer.nextToken tokenizer public long nextLong(){
            return Long.parseLong(next());
        } nextLong {
            return Long.parseLong(next());
        } return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next public class Main{

    public static void main(String[] args) throws Exception{

        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);

        Task solver = new Task();
        solver.solve(in,out);
        out.close();
    }

    static class Task{

        public void solve(InputReader in ,PrintWriter out){

            long a = in.nextLong();
            long b = in.nextLong();
            long mid = (a+b)/2;
            if(Math.abs(a - mid) == Math.abs(b - mid))
              out.println(mid);
            else 
              out.println("IMPOSSIBLE");
        }
    }

    static class InputReader{

        BufferedReader reader;
        StringTokenizer tokenizer;

        public InputReader(InputStream inputStream){
            reader = new BufferedReader(new InputStreamReader(inputStream));
            tokenizer = null;
        }

        public String next(){
            while(tokenizer == null || !tokenizer.hasMoreTokens()){
                try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public long nextLong(){
            return Long.parseLong(next());
        }
    }
} public class Main{

    public static void main(String[] args) throws Exception{

        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);

        Task solver = new Task();
        solver.solve(in,out);
        out.close();
    }

    static class Task{

        public void solve(InputReader in ,PrintWriter out){

            long a = in.nextLong();
            long b = in.nextLong();
            long mid = (a+b)/2;
            if(Math.abs(a - mid) == Math.abs(b - mid))
              out.println(mid);
            else 
              out.println("IMPOSSIBLE");
        }
    }

    static class InputReader{

        BufferedReader reader;
        StringTokenizer tokenizer;

        public InputReader(InputStream inputStream){
            reader = new BufferedReader(new InputStreamReader(inputStream));
            tokenizer = null;
        }

        public String next(){
            while(tokenizer == null || !tokenizer.hasMoreTokens()){
                try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public long nextLong(){
            return Long.parseLong(next());
        }
    }
} Main
import java.util.*;
import java.io.*;

public class Main{

    public static void main(String[] args) throws Exception{

        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);

        Task solver = new Task();
        solver.solve(in,out);
        out.close();
    }

    static class Task{

        public void solve(InputReader in ,PrintWriter out){

            long a = in.nextLong();
            long b = in.nextLong();
            long mid = (a+b)/2;
            if(Math.abs(a - mid) == Math.abs(b - mid))
              out.println(mid);
            else 
              out.println("IMPOSSIBLE");
        }
    }

    static class InputReader{

        BufferedReader reader;
        StringTokenizer tokenizer;

        public InputReader(InputStream inputStream){
            reader = new BufferedReader(new InputStreamReader(inputStream));
            tokenizer = null;
        }

        public String next(){
            while(tokenizer == null || !tokenizer.hasMoreTokens()){
                try{
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch(IOException e){
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public long nextLong(){
            return Long.parseLong(next());
        }
    }
}