import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		int k = a + b;
        if(k%2==0)
            System.out.println(k/2);
        else
            System.out.println("IMPOSSIBLE");
    }
} import java.util.Scanner; Scanner java.util public class Main{
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		int k = a + b;
        if(k%2==0)
            System.out.println(k/2);
        else
            System.out.println("IMPOSSIBLE");
    }
} Main public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		int k = a + b;
        if(k%2==0)
            System.out.println(k/2);
        else
            System.out.println("IMPOSSIBLE");
    } main {
		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		int k = a + b;
        if(k%2==0)
            System.out.println(k/2);
        else
            System.out.println("IMPOSSIBLE");
    } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int a = scan.nextInt(); a scan.nextInt() scan.nextInt scan int b = scan.nextInt(); b scan.nextInt() scan.nextInt scan int k = a + b; k a + b a b if(k%2==0)
            System.out.println(k/2);
        else
            System.out.println("IMPOSSIBLE"); k%2==0 k%2 k 2 0 System.out.println(k/2) System.out.println System.out System System.out k/2 k 2 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 scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		int k = a + b;
        if(k%2==0)
            System.out.println(k/2);
        else
            System.out.println("IMPOSSIBLE");
    }
} public class Main{
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		int k = a + b;
        if(k%2==0)
            System.out.println(k/2);
        else
            System.out.println("IMPOSSIBLE");
    }
} Main
import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		int k = a + b;
        if(k%2==0)
            System.out.println(k/2);
        else
            System.out.println("IMPOSSIBLE");
    }
}