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 c = Math.max(a-b,b-a);
        if(c%2==0){System.out.println(Math.max(a,b)-c/2);}
		else{System.out.println("IMPOSSIBLE");}
	}
} 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 c = Math.max(a-b,b-a);
        if(c%2==0){System.out.println(Math.max(a,b)-c/2);}
		else{System.out.println("IMPOSSIBLE");}
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
        int b = sc.nextInt();
		int c = Math.max(a-b,b-a);
        if(c%2==0){System.out.println(Math.max(a,b)-c/2);}
		else{System.out.println("IMPOSSIBLE");}
	} main {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
        int b = sc.nextInt();
		int c = Math.max(a-b,b-a);
        if(c%2==0){System.out.println(Math.max(a,b)-c/2);}
		else{System.out.println("IMPOSSIBLE");}
	} 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 c = Math.max(a-b,b-a); c Math.max(a-b,b-a) Math.max Math a-b a b b-a b a if(c%2==0){System.out.println(Math.max(a,b)-c/2);}
		else{System.out.println("IMPOSSIBLE");} c%2==0 c%2 c 2 0 {System.out.println(Math.max(a,b)-c/2);} System.out.println(Math.max(a,b)-c/2) System.out.println System.out System System.out Math.max(a,b)-c/2 Math.max(a,b) Math.max Math a b c/2 c 2 {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 sc = new Scanner(System.in);
		int a = sc.nextInt();
        int b = sc.nextInt();
		int c = Math.max(a-b,b-a);
        if(c%2==0){System.out.println(Math.max(a,b)-c/2);}
		else{System.out.println("IMPOSSIBLE");}
	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
        int b = sc.nextInt();
		int c = Math.max(a-b,b-a);
        if(c%2==0){System.out.println(Math.max(a,b)-c/2);}
		else{System.out.println("IMPOSSIBLE");}
	}
} 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 c = Math.max(a-b,b-a);
        if(c%2==0){System.out.println(Math.max(a,b)-c/2);}
		else{System.out.println("IMPOSSIBLE");}
	}
}
