import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner reader = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int A, B, x;
		A = reader.nextInt();
		B = reader.nextInt();
		x = A + B;
		if((x & 1) == 0)
			System.out.println(x/2);
		else
			System.out.println("IMPOSSIBLE");
	}

} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner reader = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int A, B, x;
		A = reader.nextInt();
		B = reader.nextInt();
		x = A + B;
		if((x & 1) == 0)
			System.out.println(x/2);
		else
			System.out.println("IMPOSSIBLE");
	}

} Main public static void main(String[] args) {
		Scanner reader = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int A, B, x;
		A = reader.nextInt();
		B = reader.nextInt();
		x = A + B;
		if((x & 1) == 0)
			System.out.println(x/2);
		else
			System.out.println("IMPOSSIBLE");
	} main {
		Scanner reader = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int A, B, x;
		A = reader.nextInt();
		B = reader.nextInt();
		x = A + B;
		if((x & 1) == 0)
			System.out.println(x/2);
		else
			System.out.println("IMPOSSIBLE");
	} Scanner reader = new Scanner(new BufferedReader(new InputStreamReader(System.in))); reader new Scanner(new BufferedReader(new InputStreamReader(System.in))) int A A B B x; x A = reader.nextInt() A reader.nextInt() reader.nextInt reader B = reader.nextInt() B reader.nextInt() reader.nextInt reader x = A + B x A + B A B if((x & 1) == 0)
			System.out.println(x/2);
		else
			System.out.println("IMPOSSIBLE"); (x & 1) == 0 (x & 1) x 1 0 System.out.println(x/2) System.out.println System.out System System.out x/2 x 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 reader = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int A, B, x;
		A = reader.nextInt();
		B = reader.nextInt();
		x = A + B;
		if((x & 1) == 0)
			System.out.println(x/2);
		else
			System.out.println("IMPOSSIBLE");
	}

} public class Main {

	public static void main(String[] args) {
		Scanner reader = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int A, B, x;
		A = reader.nextInt();
		B = reader.nextInt();
		x = A + B;
		if((x & 1) == 0)
			System.out.println(x/2);
		else
			System.out.println("IMPOSSIBLE");
	}

} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner reader = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int A, B, x;
		A = reader.nextInt();
		B = reader.nextInt();
		x = A + B;
		if((x & 1) == 0)
			System.out.println(x/2);
		else
			System.out.println("IMPOSSIBLE");
	}

}
