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 x = a%2;
  int y = b%2;
  if(a>b){
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
  }else{
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }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 x = a%2;
  int y = b%2;
  if(a>b){
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
  }else{
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }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 x = a%2;
  int y = b%2;
  if(a>b){
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
  }else{
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
    }
  } main {
  Scanner sc = new Scanner(System.in);
  int a = sc.nextInt();
  int b = sc.nextInt();
  int x = a%2;
  int y = b%2;
  if(a>b){
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
  }else{
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }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 x = a%2; x a%2 a 2 int y = b%2; y b%2 b 2 if(a>b){
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
  }else{
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
    } a>b a b {
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
  } if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      } (x==0 && y==0)||(x!=0 && y!=0) (x==0 && y==0) x==0 x 0 y==0 y 0 (x!=0 && y!=0) x!=0 x 0 y!=0 y 0 {
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      } int ak = (a-b)/2; ak (a-b)/2 (a-b) a b 2 int k = a-ak; k a-ak a ak System.out.println(k) System.out.println System.out System System.out k {
        System.out.println("IMPOSSIBLE");
      } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
    } if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      } (x==0 && y==0)||(x!=0 && y!=0) (x==0 && y==0) x==0 x 0 y==0 y 0 (x!=0 && y!=0) x!=0 x 0 y!=0 y 0 {
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      } int bk = (b-a)/2; bk (b-a)/2 (b-a) b a 2 int k = b-bk; k b-bk b bk System.out.println(k) System.out.println System.out System System.out k {
        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 x = a%2;
  int y = b%2;
  if(a>b){
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
  }else{
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }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 x = a%2;
  int y = b%2;
  if(a>b){
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
  }else{
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }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 x = a%2;
  int y = b%2;
  if(a>b){
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
  }else{
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
    }
  }
}