import java.io.*;
import java.util.ArrayList;
public class Main{
  public static void main(String args[]){
    String str;
          int limit=(int)Math.pow(10,9);
    try{
      BufferedReader br=new BufferedReader(new InputStreamReader(System.in,"UTF-8"));
      str=br.readLine();
      String[] numstr = str.split(" ");
      int[] num=new int[2];
      for(int i=0;i<2;i++){
        num[i]=Integer.parseInt(numstr[i]);
      }
      int ans=calAbs(num);
      if(ans==limit){
        System.out.println("IMPOSSIBLE");
      }else{
      System.out.println(ans);
    }
    }catch(IOException e){
      e.printStackTrace();
    }
  }

    public static int calAbs(int[] num){
      int i=Integer.MIN_VALUE;
      int a,b;
      int limit=(int)Math.pow(10,9);
      if((num[0]+num[1])%2!=0)return limit;
      int ave=(num[0]+num[1])/2;
      return ave;
    }

}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz