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 diss = Math.max(a, b) - Math.min(a, b);

        if (diss % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            int half = diss / 2;
            System.out.println(Math.min(a, b) + half);
        }
	}
} 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 diss = Math.max(a, b) - Math.min(a, b);

        if (diss % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            int half = diss / 2;
            System.out.println(Math.min(a, b) + half);
        }
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
        
        int a = sc.nextInt();
        int b = sc.nextInt();
        int diss = Math.max(a, b) - Math.min(a, b);

        if (diss % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            int half = diss / 2;
            System.out.println(Math.min(a, b) + half);
        }
	} main {
		Scanner sc = new Scanner(System.in);
        
        int a = sc.nextInt();
        int b = sc.nextInt();
        int diss = Math.max(a, b) - Math.min(a, b);

        if (diss % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            int half = diss / 2;
            System.out.println(Math.min(a, b) + half);
        }
	} 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 diss = Math.max(a, b) - Math.min(a, b); diss Math.max(a, b) - Math.min(a, b) Math.max(a, b) Math.max Math a b Math.min(a, b) Math.min Math a b if (diss % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            int half = diss / 2;
            System.out.println(Math.min(a, b) + half);
        } diss % 2 == 1 diss % 2 diss 2 1 {
            System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
            int half = diss / 2;
            System.out.println(Math.min(a, b) + half);
        } int half = diss / 2; half diss / 2 diss 2 System.out.println(Math.min(a, b) + half) System.out.println System.out System System.out Math.min(a, b) + half Math.min(a, b) Math.min Math a b half 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 diss = Math.max(a, b) - Math.min(a, b);

        if (diss % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            int half = diss / 2;
            System.out.println(Math.min(a, b) + half);
        }
	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
        
        int a = sc.nextInt();
        int b = sc.nextInt();
        int diss = Math.max(a, b) - Math.min(a, b);

        if (diss % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            int half = diss / 2;
            System.out.println(Math.min(a, b) + half);
        }
	}
} 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 diss = Math.max(a, b) - Math.min(a, b);

        if (diss % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            int half = diss / 2;
            System.out.println(Math.min(a, b) + half);
        }
	}
}
