import java.util.*;

public class Main{
  public static void main(String args[]){
    Scanner scanner = new Scanner(System.in);
    int A = scanner.nextInt();
    int B = scanner.nextInt();
    
    int K;
    boolean flg = false;
    
    /*
    * A <= B のとき A <= K <= B,
    * A > B のとき B < K < A でなければならない
    */
    
    if(A <= B){
      for(K = A; K <= B; K++){
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
      
    } else {
      for(K = B + 1; K < A; K++){
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
    }
    
    if(flg == false){
      System.out.println("IMPOSSIBLE");
    }
  }
} import java.util.*; util.* java public class Main{
  public static void main(String args[]){
    Scanner scanner = new Scanner(System.in);
    int A = scanner.nextInt();
    int B = scanner.nextInt();
    
    int K;
    boolean flg = false;
    
    /*
    * A <= B のとき A <= K <= B,
    * A > B のとき B < K < A でなければならない
    */
    
    if(A <= B){
      for(K = A; K <= B; K++){
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
      
    } else {
      for(K = B + 1; K < A; K++){
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
    }
    
    if(flg == false){
      System.out.println("IMPOSSIBLE");
    }
  }
} Main public static void main(String args[]){
    Scanner scanner = new Scanner(System.in);
    int A = scanner.nextInt();
    int B = scanner.nextInt();
    
    int K;
    boolean flg = false;
    
    /*
    * A <= B のとき A <= K <= B,
    * A > B のとき B < K < A でなければならない
    */
    
    if(A <= B){
      for(K = A; K <= B; K++){
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
      
    } else {
      for(K = B + 1; K < A; K++){
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
    }
    
    if(flg == false){
      System.out.println("IMPOSSIBLE");
    }
  } main {
    Scanner scanner = new Scanner(System.in);
    int A = scanner.nextInt();
    int B = scanner.nextInt();
    
    int K;
    boolean flg = false;
    
    /*
    * A <= B のとき A <= K <= B,
    * A > B のとき B < K < A でなければならない
    */
    
    if(A <= B){
      for(K = A; K <= B; K++){
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
      
    } else {
      for(K = B + 1; K < A; K++){
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
    }
    
    if(flg == false){
      System.out.println("IMPOSSIBLE");
    }
  } Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int A = scanner.nextInt(); A scanner.nextInt() scanner.nextInt scanner int B = scanner.nextInt(); B scanner.nextInt() scanner.nextInt scanner int K; K boolean flg = false; flg false if(A <= B){
      for(K = A; K <= B; K++){
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
      
    } else {
      for(K = B + 1; K < A; K++){
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
    } A <= B A B {
      for(K = A; K <= B; K++){
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
      
    } for(K = A; K <= B; K++){
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      } K = A; K = A K A K <= B K B K++ K++ K {
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      } {
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      } int minus = -1 * (A - K); minus -1 * (A - K) -1 1 (A - K) A K int plus = B - K; plus B - K B K if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        } minus == plus minus plus {
          System.out.println(K);
          flg = true;
          break;
        } System.out.println(K) System.out.println System.out System System.out K flg = true flg true break; {
      for(K = B + 1; K < A; K++){
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
    } for(K = B + 1; K < A; K++){
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      } K = B + 1; K = B + 1 K B + 1 B 1 K < A K A K++ K++ K {
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      } {
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      } int minus = -1 * (B - K); minus -1 * (B - K) -1 1 (B - K) B K int plus = A - K; plus A - K A K if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        } minus == plus minus plus {
          System.out.println(K);
          flg = true;
          break;
        } System.out.println(K) System.out.println System.out System System.out K flg = true flg true break; if(flg == false){
      System.out.println("IMPOSSIBLE");
    } flg == false flg false {
      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 scanner = new Scanner(System.in);
    int A = scanner.nextInt();
    int B = scanner.nextInt();
    
    int K;
    boolean flg = false;
    
    /*
    * A <= B のとき A <= K <= B,
    * A > B のとき B < K < A でなければならない
    */
    
    if(A <= B){
      for(K = A; K <= B; K++){
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
      
    } else {
      for(K = B + 1; K < A; K++){
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
    }
    
    if(flg == false){
      System.out.println("IMPOSSIBLE");
    }
  }
} public class Main{
  public static void main(String args[]){
    Scanner scanner = new Scanner(System.in);
    int A = scanner.nextInt();
    int B = scanner.nextInt();
    
    int K;
    boolean flg = false;
    
    /*
    * A <= B のとき A <= K <= B,
    * A > B のとき B < K < A でなければならない
    */
    
    if(A <= B){
      for(K = A; K <= B; K++){
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
      
    } else {
      for(K = B + 1; K < A; K++){
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
    }
    
    if(flg == false){
      System.out.println("IMPOSSIBLE");
    }
  }
} Main
import java.util.*;

public class Main{
  public static void main(String args[]){
    Scanner scanner = new Scanner(System.in);
    int A = scanner.nextInt();
    int B = scanner.nextInt();
    
    int K;
    boolean flg = false;
    
    /*
    * A <= B のとき A <= K <= B,
    * A > B のとき B < K < A でなければならない
    */
    
    if(A <= B){
      for(K = A; K <= B; K++){
        int minus = -1 * (A - K);
        int plus = B - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
      
    } else {
      for(K = B + 1; K < A; K++){
        int minus = -1 * (B - K);
        int plus = A - K;
        
        if(minus == plus){
          System.out.println(K);
          flg = true;
          break;
        }
      }
    }
    
    if(flg == false){
      System.out.println("IMPOSSIBLE");
    }
  }
}