import java.util.Scanner;

public class Main { 

	public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	String[] line = sc.nextLine().split(" ");
    	long firstNumber = Long.parseLong(line[0]);
    	long secoundNumber = Long.parseLong(line[1]);
    	String result = "";
    	if(firstNumber % 2 != secoundNumber % 2 ) {
    		result = "IMPOSSIBLE";
    	}else {
    		long tmp = (firstNumber + secoundNumber)/2;
    		result = String.valueOf(tmp);
    	}
    	
    	
    	System.out.println(result);
    	
	}
} import java.util.Scanner; Scanner java.util public class Main { 

	public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	String[] line = sc.nextLine().split(" ");
    	long firstNumber = Long.parseLong(line[0]);
    	long secoundNumber = Long.parseLong(line[1]);
    	String result = "";
    	if(firstNumber % 2 != secoundNumber % 2 ) {
    		result = "IMPOSSIBLE";
    	}else {
    		long tmp = (firstNumber + secoundNumber)/2;
    		result = String.valueOf(tmp);
    	}
    	
    	
    	System.out.println(result);
    	
	}
} Main public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	String[] line = sc.nextLine().split(" ");
    	long firstNumber = Long.parseLong(line[0]);
    	long secoundNumber = Long.parseLong(line[1]);
    	String result = "";
    	if(firstNumber % 2 != secoundNumber % 2 ) {
    		result = "IMPOSSIBLE";
    	}else {
    		long tmp = (firstNumber + secoundNumber)/2;
    		result = String.valueOf(tmp);
    	}
    	
    	
    	System.out.println(result);
    	
	} main {
    	Scanner sc = new Scanner(System.in);
    	String[] line = sc.nextLine().split(" ");
    	long firstNumber = Long.parseLong(line[0]);
    	long secoundNumber = Long.parseLong(line[1]);
    	String result = "";
    	if(firstNumber % 2 != secoundNumber % 2 ) {
    		result = "IMPOSSIBLE";
    	}else {
    		long tmp = (firstNumber + secoundNumber)/2;
    		result = String.valueOf(tmp);
    	}
    	
    	
    	System.out.println(result);
    	
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String[] line = sc.nextLine().split(" "); line sc.nextLine().split(" ") sc.nextLine().split sc.nextLine() sc.nextLine sc " " long firstNumber = Long.parseLong(line[0]); firstNumber Long.parseLong(line[0]) Long.parseLong Long line[0] line 0 long secoundNumber = Long.parseLong(line[1]); secoundNumber Long.parseLong(line[1]) Long.parseLong Long line[1] line 1 String result = ""; result "" if(firstNumber % 2 != secoundNumber % 2 ) {
    		result = "IMPOSSIBLE";
    	}else {
    		long tmp = (firstNumber + secoundNumber)/2;
    		result = String.valueOf(tmp);
    	} firstNumber % 2 != secoundNumber % 2 firstNumber % 2 firstNumber 2 secoundNumber % 2 secoundNumber 2 {
    		result = "IMPOSSIBLE";
    	} result = "IMPOSSIBLE" result "IMPOSSIBLE" {
    		long tmp = (firstNumber + secoundNumber)/2;
    		result = String.valueOf(tmp);
    	} long tmp = (firstNumber + secoundNumber)/2; tmp (firstNumber + secoundNumber)/2 (firstNumber + secoundNumber) firstNumber secoundNumber 2 result = String.valueOf(tmp) result String.valueOf(tmp) String.valueOf String tmp System.out.println(result) System.out.println System.out System System.out result String[] args args public class Main { 

	public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	String[] line = sc.nextLine().split(" ");
    	long firstNumber = Long.parseLong(line[0]);
    	long secoundNumber = Long.parseLong(line[1]);
    	String result = "";
    	if(firstNumber % 2 != secoundNumber % 2 ) {
    		result = "IMPOSSIBLE";
    	}else {
    		long tmp = (firstNumber + secoundNumber)/2;
    		result = String.valueOf(tmp);
    	}
    	
    	
    	System.out.println(result);
    	
	}
} public class Main { 

	public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	String[] line = sc.nextLine().split(" ");
    	long firstNumber = Long.parseLong(line[0]);
    	long secoundNumber = Long.parseLong(line[1]);
    	String result = "";
    	if(firstNumber % 2 != secoundNumber % 2 ) {
    		result = "IMPOSSIBLE";
    	}else {
    		long tmp = (firstNumber + secoundNumber)/2;
    		result = String.valueOf(tmp);
    	}
    	
    	
    	System.out.println(result);
    	
	}
} Main
import java.util.Scanner;

public class Main { 

	public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	String[] line = sc.nextLine().split(" ");
    	long firstNumber = Long.parseLong(line[0]);
    	long secoundNumber = Long.parseLong(line[1]);
    	String result = "";
    	if(firstNumber % 2 != secoundNumber % 2 ) {
    		result = "IMPOSSIBLE";
    	}else {
    		long tmp = (firstNumber + secoundNumber)/2;
    		result = String.valueOf(tmp);
    	}
    	
    	
    	System.out.println(result);
    	
	}
}