import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		long A = sc.nextLong(),B = sc.nextLong();
		double k = 0;
		k = (A+B)/2;

		if((A+B)%2==0) {
			System.out.println((int)k);
		}else {
			System.out.println("IMPOSSIBLE");
		}
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		long A = sc.nextLong(),B = sc.nextLong();
		double k = 0;
		k = (A+B)/2;

		if((A+B)%2==0) {
			System.out.println((int)k);
		}else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		long A = sc.nextLong(),B = sc.nextLong();
		double k = 0;
		k = (A+B)/2;

		if((A+B)%2==0) {
			System.out.println((int)k);
		}else {
			System.out.println("IMPOSSIBLE");
		}
	} main {
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		long A = sc.nextLong(),B = sc.nextLong();
		double k = 0;
		k = (A+B)/2;

		if((A+B)%2==0) {
			System.out.println((int)k);
		}else {
			System.out.println("IMPOSSIBLE");
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long A = sc.nextLong() A sc.nextLong() sc.nextLong sc B = sc.nextLong(); B sc.nextLong() sc.nextLong sc double k = 0; k 0 k = (A+B)/2 k (A+B)/2 (A+B) A B 2 if((A+B)%2==0) {
			System.out.println((int)k);
		}else {
			System.out.println("IMPOSSIBLE");
		} (A+B)%2==0 (A+B)%2 (A+B) A B 2 0 {
			System.out.println((int)k);
		} System.out.println((int)k) System.out.println System.out System System.out (int)k {
			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);
		// 整数の入力
		long A = sc.nextLong(),B = sc.nextLong();
		double k = 0;
		k = (A+B)/2;

		if((A+B)%2==0) {
			System.out.println((int)k);
		}else {
			System.out.println("IMPOSSIBLE");
		}
	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		long A = sc.nextLong(),B = sc.nextLong();
		double k = 0;
		k = (A+B)/2;

		if((A+B)%2==0) {
			System.out.println((int)k);
		}else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main
import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		long A = sc.nextLong(),B = sc.nextLong();
		double k = 0;
		k = (A+B)/2;

		if((A+B)%2==0) {
			System.out.println((int)k);
		}else {
			System.out.println("IMPOSSIBLE");
		}
	}
}
