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

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

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

	} main {
		int a , b;
		Scanner input = new Scanner(System.in);
		a = input.nextInt();
		b = input.nextInt();
		if((a+b)%2 == 0)
			System.out.println((a+b)/2);
		else
			System.out.println("IMPOSSIBLE");

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

	}
}