import java.util.*;

public class Main{
    public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);

      int A=sc.nextInt();
      int B=sc.nextInt();

      int C=A+B;

      int c=(A+B)/2;

      int ans=0;

      for(int k=0;k<=c;k++){
        if(C==2*k){
          ans=k;
        }
      }

      if(ans!=0){
        System.out.println(ans);
      }
      else{
        System.out.println("IMPOSSIBLE");
      }

    }
} import java.util.*; util.* java public class Main{
    public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);

      int A=sc.nextInt();
      int B=sc.nextInt();

      int C=A+B;

      int c=(A+B)/2;

      int ans=0;

      for(int k=0;k<=c;k++){
        if(C==2*k){
          ans=k;
        }
      }

      if(ans!=0){
        System.out.println(ans);
      }
      else{
        System.out.println("IMPOSSIBLE");
      }

    }
} Main public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);

      int A=sc.nextInt();
      int B=sc.nextInt();

      int C=A+B;

      int c=(A+B)/2;

      int ans=0;

      for(int k=0;k<=c;k++){
        if(C==2*k){
          ans=k;
        }
      }

      if(ans!=0){
        System.out.println(ans);
      }
      else{
        System.out.println("IMPOSSIBLE");
      }

    } main {
      Scanner sc = new Scanner(System.in);

      int A=sc.nextInt();
      int B=sc.nextInt();

      int C=A+B;

      int c=(A+B)/2;

      int ans=0;

      for(int k=0;k<=c;k++){
        if(C==2*k){
          ans=k;
        }
      }

      if(ans!=0){
        System.out.println(ans);
      }
      else{
        System.out.println("IMPOSSIBLE");
      }

    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int A=sc.nextInt(); A sc.nextInt() sc.nextInt sc int B=sc.nextInt(); B sc.nextInt() sc.nextInt sc int C=A+B; C A+B A B int c=(A+B)/2; c (A+B)/2 (A+B) A B 2 int ans=0; ans 0 for(int k=0;k<=c;k++){
        if(C==2*k){
          ans=k;
        }
      } int k=0; int k=0; k 0 k<=c k c k++ k++ k {
        if(C==2*k){
          ans=k;
        }
      } {
        if(C==2*k){
          ans=k;
        }
      } if(C==2*k){
          ans=k;
        } C==2*k C 2*k 2 k {
          ans=k;
        } ans=k ans k if(ans!=0){
        System.out.println(ans);
      }
      else{
        System.out.println("IMPOSSIBLE");
      } ans!=0 ans 0 {
        System.out.println(ans);
      } System.out.println(ans) System.out.println System.out System System.out ans {
        System.out.println("IMPOSSIBLE");
      } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args public class Main{
    public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);

      int A=sc.nextInt();
      int B=sc.nextInt();

      int C=A+B;

      int c=(A+B)/2;

      int ans=0;

      for(int k=0;k<=c;k++){
        if(C==2*k){
          ans=k;
        }
      }

      if(ans!=0){
        System.out.println(ans);
      }
      else{
        System.out.println("IMPOSSIBLE");
      }

    }
} public class Main{
    public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);

      int A=sc.nextInt();
      int B=sc.nextInt();

      int C=A+B;

      int c=(A+B)/2;

      int ans=0;

      for(int k=0;k<=c;k++){
        if(C==2*k){
          ans=k;
        }
      }

      if(ans!=0){
        System.out.println(ans);
      }
      else{
        System.out.println("IMPOSSIBLE");
      }

    }
} Main
import java.util.*;

public class Main{
    public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);

      int A=sc.nextInt();
      int B=sc.nextInt();

      int C=A+B;

      int c=(A+B)/2;

      int ans=0;

      for(int k=0;k<=c;k++){
        if(C==2*k){
          ans=k;
        }
      }

      if(ans!=0){
        System.out.println(ans);
      }
      else{
        System.out.println("IMPOSSIBLE");
      }

    }
}
