import java.util.*;
class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long ans = sc.nextLong() + sc.nextLong();
    if (ans%2 == 1)
      System.out.println("IMPOSSIBLE");
    else
    System.out.println(ans/2);
  }
} import java.util.*; util.* java class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long ans = sc.nextLong() + sc.nextLong();
    if (ans%2 == 1)
      System.out.println("IMPOSSIBLE");
    else
    System.out.println(ans/2);
  }
} Main public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long ans = sc.nextLong() + sc.nextLong();
    if (ans%2 == 1)
      System.out.println("IMPOSSIBLE");
    else
    System.out.println(ans/2);
  } main {
    Scanner sc = new Scanner(System.in);
    long ans = sc.nextLong() + sc.nextLong();
    if (ans%2 == 1)
      System.out.println("IMPOSSIBLE");
    else
    System.out.println(ans/2);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long ans = sc.nextLong() + sc.nextLong(); ans sc.nextLong() + sc.nextLong() sc.nextLong() sc.nextLong sc sc.nextLong() sc.nextLong sc if (ans%2 == 1)
      System.out.println("IMPOSSIBLE");
    else
    System.out.println(ans/2); ans%2 == 1 ans%2 ans 2 1 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" System.out.println(ans/2) System.out.println System.out System System.out ans/2 ans 2 String[] args args
import java.util.*;
class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long ans = sc.nextLong() + sc.nextLong();
    if (ans%2 == 1)
      System.out.println("IMPOSSIBLE");
    else
    System.out.println(ans/2);
  }
}
