import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();
		int K = (A + B) / 2;

		if (A % 2 == 0 && B % 2 == 0) {
			System.out.println(K);
		} else if (A % 2 == 1 && B % 2 == 1) {
			System.out.println(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);

		int A = sc.nextInt();
		int B = sc.nextInt();
		int K = (A + B) / 2;

		if (A % 2 == 0 && B % 2 == 0) {
			System.out.println(K);
		} else if (A % 2 == 1 && B % 2 == 1) {
			System.out.println(K);
		} 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 K = (A + B) / 2;

		if (A % 2 == 0 && B % 2 == 0) {
			System.out.println(K);
		} else if (A % 2 == 1 && B % 2 == 1) {
			System.out.println(K);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	} main {
		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();
		int K = (A + B) / 2;

		if (A % 2 == 0 && B % 2 == 0) {
			System.out.println(K);
		} else if (A % 2 == 1 && B % 2 == 1) {
			System.out.println(K);
		} 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 K = (A + B) / 2; K (A + B) / 2 (A + B) A B 2 if (A % 2 == 0 && B % 2 == 0) {
			System.out.println(K);
		} else if (A % 2 == 1 && B % 2 == 1) {
			System.out.println(K);
		} else {
			System.out.println("IMPOSSIBLE");
		} A % 2 == 0 && B % 2 == 0 A % 2 == 0 A % 2 A 2 0 B % 2 == 0 B % 2 B 2 0 {
			System.out.println(K);
		} System.out.println(K) System.out.println System.out System System.out K if (A % 2 == 1 && B % 2 == 1) {
			System.out.println(K);
		} else {
			System.out.println("IMPOSSIBLE");
		} A % 2 == 1 && B % 2 == 1 A % 2 == 1 A % 2 A 2 1 B % 2 == 1 B % 2 B 2 1 {
			System.out.println(K);
		} System.out.println(K) System.out.println System.out System System.out 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);

		int A = sc.nextInt();
		int B = sc.nextInt();
		int K = (A + B) / 2;

		if (A % 2 == 0 && B % 2 == 0) {
			System.out.println(K);
		} else if (A % 2 == 1 && B % 2 == 1) {
			System.out.println(K);
		} 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 K = (A + B) / 2;

		if (A % 2 == 0 && B % 2 == 0) {
			System.out.println(K);
		} else if (A % 2 == 1 && B % 2 == 1) {
			System.out.println(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);

		int A = sc.nextInt();
		int B = sc.nextInt();
		int K = (A + B) / 2;

		if (A % 2 == 0 && B % 2 == 0) {
			System.out.println(K);
		} else if (A % 2 == 1 && B % 2 == 1) {
			System.out.println(K);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
}