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

public class Main
{
	PrintWriter out = new PrintWriter(System.out);
	BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer tok = new StringTokenizer("");
    String next() throws IOException {
        if (!tok.hasMoreTokens()) { tok = new StringTokenizer(in.readLine()); }
        return tok.nextToken();
    }
    int ni() throws IOException { return Integer.parseInt(next()); }
    long nl() throws IOException { return Long.parseLong(next()); }
  
    void solve() throws IOException {
        int n=ni(),k=ni();
        if ((n+k)%2==1) System.out.println("IMPOSSIBLE");
        else System.out.println((n+k)/2);
    }
    
    public static void main(String[] args) throws IOException {
        new Main().solve();
    }
} import java.util.*; util.* java import java.lang.*; lang.* java import java.io.*; io.* java public class Main
{
	PrintWriter out = new PrintWriter(System.out);
	BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer tok = new StringTokenizer("");
    String next() throws IOException {
        if (!tok.hasMoreTokens()) { tok = new StringTokenizer(in.readLine()); }
        return tok.nextToken();
    }
    int ni() throws IOException { return Integer.parseInt(next()); }
    long nl() throws IOException { return Long.parseLong(next()); }
  
    void solve() throws IOException {
        int n=ni(),k=ni();
        if ((n+k)%2==1) System.out.println("IMPOSSIBLE");
        else System.out.println((n+k)/2);
    }
    
    public static void main(String[] args) throws IOException {
        new Main().solve();
    }
} Main PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); in new BufferedReader(new InputStreamReader(System.in)) StringTokenizer tok = new StringTokenizer(""); tok new StringTokenizer("") String next() throws IOException {
        if (!tok.hasMoreTokens()) { tok = new StringTokenizer(in.readLine()); }
        return tok.nextToken();
    } next {
        if (!tok.hasMoreTokens()) { tok = new StringTokenizer(in.readLine()); }
        return tok.nextToken();
    } if (!tok.hasMoreTokens()) { tok = new StringTokenizer(in.readLine()); } !tok.hasMoreTokens() tok.hasMoreTokens() tok.hasMoreTokens tok { tok = new StringTokenizer(in.readLine()); } tok = new StringTokenizer(in.readLine()) tok new StringTokenizer(in.readLine()) return tok.nextToken(); tok.nextToken() tok.nextToken tok int ni() throws IOException { return Integer.parseInt(next()); } ni { return Integer.parseInt(next()); } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next long nl() throws IOException { return Long.parseLong(next()); } nl { return Long.parseLong(next()); } return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next void solve() throws IOException {
        int n=ni(),k=ni();
        if ((n+k)%2==1) System.out.println("IMPOSSIBLE");
        else System.out.println((n+k)/2);
    } solve {
        int n=ni(),k=ni();
        if ((n+k)%2==1) System.out.println("IMPOSSIBLE");
        else System.out.println((n+k)/2);
    } int n=ni() n ni() ni k=ni(); k ni() ni if ((n+k)%2==1) System.out.println("IMPOSSIBLE");
        else System.out.println((n+k)/2); (n+k)%2==1 (n+k)%2 (n+k) n k 2 1 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" System.out.println((n+k)/2) System.out.println System.out System System.out (n+k)/2 (n+k) n k 2 public static void main(String[] args) throws IOException {
        new Main().solve();
    } main {
        new Main().solve();
    } new Main().solve() new Main().solve new Main() String[] args args public class Main
{
	PrintWriter out = new PrintWriter(System.out);
	BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer tok = new StringTokenizer("");
    String next() throws IOException {
        if (!tok.hasMoreTokens()) { tok = new StringTokenizer(in.readLine()); }
        return tok.nextToken();
    }
    int ni() throws IOException { return Integer.parseInt(next()); }
    long nl() throws IOException { return Long.parseLong(next()); }
  
    void solve() throws IOException {
        int n=ni(),k=ni();
        if ((n+k)%2==1) System.out.println("IMPOSSIBLE");
        else System.out.println((n+k)/2);
    }
    
    public static void main(String[] args) throws IOException {
        new Main().solve();
    }
} public class Main
{
	PrintWriter out = new PrintWriter(System.out);
	BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer tok = new StringTokenizer("");
    String next() throws IOException {
        if (!tok.hasMoreTokens()) { tok = new StringTokenizer(in.readLine()); }
        return tok.nextToken();
    }
    int ni() throws IOException { return Integer.parseInt(next()); }
    long nl() throws IOException { return Long.parseLong(next()); }
  
    void solve() throws IOException {
        int n=ni(),k=ni();
        if ((n+k)%2==1) System.out.println("IMPOSSIBLE");
        else System.out.println((n+k)/2);
    }
    
    public static void main(String[] args) throws IOException {
        new Main().solve();
    }
} Main
import java.util.*;
import java.lang.*;
import java.io.*;

public class Main
{
	PrintWriter out = new PrintWriter(System.out);
	BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer tok = new StringTokenizer("");
    String next() throws IOException {
        if (!tok.hasMoreTokens()) { tok = new StringTokenizer(in.readLine()); }
        return tok.nextToken();
    }
    int ni() throws IOException { return Integer.parseInt(next()); }
    long nl() throws IOException { return Long.parseLong(next()); }
  
    void solve() throws IOException {
        int n=ni(),k=ni();
        if ((n+k)%2==1) System.out.println("IMPOSSIBLE");
        else System.out.println((n+k)/2);
    }
    
    public static void main(String[] args) throws IOException {
        new Main().solve();
    }
}