import java.util.Scanner;

public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		int workNum[] = new int[2];
		workNum[0] = sc.nextInt();
		workNum[1] = sc.nextInt();

		int diff = workNum[0] - workNum[1];
		diff = (diff < 0) ? diff * -1 : diff;
		if(diff % 2 == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			diff /= 2;
			System.out.println(workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff);
		}
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		int workNum[] = new int[2];
		workNum[0] = sc.nextInt();
		workNum[1] = sc.nextInt();

		int diff = workNum[0] - workNum[1];
		diff = (diff < 0) ? diff * -1 : diff;
		if(diff % 2 == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			diff /= 2;
			System.out.println(workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff);
		}
	}
} Main public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		int workNum[] = new int[2];
		workNum[0] = sc.nextInt();
		workNum[1] = sc.nextInt();

		int diff = workNum[0] - workNum[1];
		diff = (diff < 0) ? diff * -1 : diff;
		if(diff % 2 == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			diff /= 2;
			System.out.println(workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff);
		}
	} main {

		Scanner sc = new Scanner(System.in);

		int workNum[] = new int[2];
		workNum[0] = sc.nextInt();
		workNum[1] = sc.nextInt();

		int diff = workNum[0] - workNum[1];
		diff = (diff < 0) ? diff * -1 : diff;
		if(diff % 2 == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			diff /= 2;
			System.out.println(workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff);
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int workNum[] = new int[2]; workNum new int[2] 2 workNum[0] = sc.nextInt() workNum[0] workNum 0 sc.nextInt() sc.nextInt sc workNum[1] = sc.nextInt() workNum[1] workNum 1 sc.nextInt() sc.nextInt sc int diff = workNum[0] - workNum[1]; diff workNum[0] - workNum[1] workNum[0] workNum 0 workNum[1] workNum 1 diff = (diff < 0) ? diff * -1 : diff diff (diff < 0) ? diff * -1 : diff (diff < 0) diff 0 diff * -1 diff -1 1 diff if(diff % 2 == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			diff /= 2;
			System.out.println(workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff);
		} diff % 2 == 1 diff % 2 diff 2 1 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			diff /= 2;
			System.out.println(workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff);
		} diff /= 2 diff 2 System.out.println(workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff) System.out.println System.out System System.out workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff workNum[0] > workNum[1] workNum[0] workNum 0 workNum[1] workNum 1 workNum[1] + diff workNum[1] workNum 1 diff workNum[0] + diff workNum[0] workNum 0 diff String args[] args public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		int workNum[] = new int[2];
		workNum[0] = sc.nextInt();
		workNum[1] = sc.nextInt();

		int diff = workNum[0] - workNum[1];
		diff = (diff < 0) ? diff * -1 : diff;
		if(diff % 2 == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			diff /= 2;
			System.out.println(workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff);
		}
	}
} public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		int workNum[] = new int[2];
		workNum[0] = sc.nextInt();
		workNum[1] = sc.nextInt();

		int diff = workNum[0] - workNum[1];
		diff = (diff < 0) ? diff * -1 : diff;
		if(diff % 2 == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			diff /= 2;
			System.out.println(workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff);
		}
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		int workNum[] = new int[2];
		workNum[0] = sc.nextInt();
		workNum[1] = sc.nextInt();

		int diff = workNum[0] - workNum[1];
		diff = (diff < 0) ? diff * -1 : diff;
		if(diff % 2 == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			diff /= 2;
			System.out.println(workNum[0] > workNum[1] ?
					workNum[1] + diff : workNum[0] + diff);
		}
	}
}
