import java.io.*;

public class Main {

  public static void main(String[] args){

    try{
      String str;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
      str = br.readLine();
String[] ss = str.split(" ");
      //int sample = Integer.parseInt(str);
      func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1]));
    } catch(IOException e){
      e.printStackTrace();
    }
  }

  public static void func(int a, int b){
    if((a+b)%2 == 0){
System.out.println(Math.abs((a+b)/2));
} else System.out.println("IMPOSSIBLE");
  }
} import java.io.*; io.* java public class Main {

  public static void main(String[] args){

    try{
      String str;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
      str = br.readLine();
String[] ss = str.split(" ");
      //int sample = Integer.parseInt(str);
      func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1]));
    } catch(IOException e){
      e.printStackTrace();
    }
  }

  public static void func(int a, int b){
    if((a+b)%2 == 0){
System.out.println(Math.abs((a+b)/2));
} else System.out.println("IMPOSSIBLE");
  }
} Main public static void main(String[] args){

    try{
      String str;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
      str = br.readLine();
String[] ss = str.split(" ");
      //int sample = Integer.parseInt(str);
      func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1]));
    } catch(IOException e){
      e.printStackTrace();
    }
  } main {

    try{
      String str;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
      str = br.readLine();
String[] ss = str.split(" ");
      //int sample = Integer.parseInt(str);
      func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1]));
    } catch(IOException e){
      e.printStackTrace();
    }
  } try{
      String str;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
      str = br.readLine();
String[] ss = str.split(" ");
      //int sample = Integer.parseInt(str);
      func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1]));
    } catch(IOException e){
      e.printStackTrace();
    } catch(IOException e){
      e.printStackTrace();
    } IOException e {
      e.printStackTrace();
    } e.printStackTrace() e.printStackTrace e {
      String str;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
      str = br.readLine();
String[] ss = str.split(" ");
      //int sample = Integer.parseInt(str);
      func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1]));
    } String str; str BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8")); br new BufferedReader(new InputStreamReader(System.in, "UTF-8")) str = br.readLine() str br.readLine() br.readLine br String[] ss = str.split(" "); ss str.split(" ") str.split str " " func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1])) func Integer.parseInt(ss[0]) Integer.parseInt Integer ss[0] ss 0 Integer.parseInt(ss[1]) Integer.parseInt Integer ss[1] ss 1 String[] args args public static void func(int a, int b){
    if((a+b)%2 == 0){
System.out.println(Math.abs((a+b)/2));
} else System.out.println("IMPOSSIBLE");
  } func {
    if((a+b)%2 == 0){
System.out.println(Math.abs((a+b)/2));
} else System.out.println("IMPOSSIBLE");
  } if((a+b)%2 == 0){
System.out.println(Math.abs((a+b)/2));
} else System.out.println("IMPOSSIBLE"); (a+b)%2 == 0 (a+b)%2 (a+b) a b 2 0 {
System.out.println(Math.abs((a+b)/2));
} System.out.println(Math.abs((a+b)/2)) System.out.println System.out System System.out Math.abs((a+b)/2) Math.abs Math (a+b)/2 (a+b) a b 2 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" int a a int b b public class Main {

  public static void main(String[] args){

    try{
      String str;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
      str = br.readLine();
String[] ss = str.split(" ");
      //int sample = Integer.parseInt(str);
      func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1]));
    } catch(IOException e){
      e.printStackTrace();
    }
  }

  public static void func(int a, int b){
    if((a+b)%2 == 0){
System.out.println(Math.abs((a+b)/2));
} else System.out.println("IMPOSSIBLE");
  }
} public class Main {

  public static void main(String[] args){

    try{
      String str;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
      str = br.readLine();
String[] ss = str.split(" ");
      //int sample = Integer.parseInt(str);
      func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1]));
    } catch(IOException e){
      e.printStackTrace();
    }
  }

  public static void func(int a, int b){
    if((a+b)%2 == 0){
System.out.println(Math.abs((a+b)/2));
} else System.out.println("IMPOSSIBLE");
  }
} Main
import java.io.*;

public class Main {

  public static void main(String[] args){

    try{
      String str;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
      str = br.readLine();
String[] ss = str.split(" ");
      //int sample = Integer.parseInt(str);
      func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1]));
    } catch(IOException e){
      e.printStackTrace();
    }
  }

  public static void func(int a, int b){
    if((a+b)%2 == 0){
System.out.println(Math.abs((a+b)/2));
} else System.out.println("IMPOSSIBLE");
  }
}
