import java.io.PrintStream;
import java.util.Scanner;

public class Main {
	static final PrintStream out = System.out;
	static Scanner sc = new Scanner(System.in);
	static final long Mod = 1000_000_007L;
	public static void main(String[] args) {
		Main main = new Main();
		main.solve();
		System.exit(0);
	}

	void solve() {
		long A = nextInt();
		long B = nextInt();
		long K = (A+B)/2;

		if(Math.abs(A-K) == Math.abs(B-K)) {
			out.println(K);
		}else{
			out.println("IMPOSSIBLE");
		}
	}

	public int[] nextIntArray(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)a[i] = nextInt();
		return a;
	}
	public long[] nextLongArray(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)a[i] = nextLong();
		return a;
	}
	public int nextInt() {
		return Integer.parseInt(next());
	}
	public long nextLong() {
		return Long.parseLong(next());
	}
	public String next() {
		return sc.next();
	}

} import java.io.PrintStream; PrintStream java.io import java.util.Scanner; Scanner java.util public class Main {
	static final PrintStream out = System.out;
	static Scanner sc = new Scanner(System.in);
	static final long Mod = 1000_000_007L;
	public static void main(String[] args) {
		Main main = new Main();
		main.solve();
		System.exit(0);
	}

	void solve() {
		long A = nextInt();
		long B = nextInt();
		long K = (A+B)/2;

		if(Math.abs(A-K) == Math.abs(B-K)) {
			out.println(K);
		}else{
			out.println("IMPOSSIBLE");
		}
	}

	public int[] nextIntArray(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)a[i] = nextInt();
		return a;
	}
	public long[] nextLongArray(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)a[i] = nextLong();
		return a;
	}
	public int nextInt() {
		return Integer.parseInt(next());
	}
	public long nextLong() {
		return Long.parseLong(next());
	}
	public String next() {
		return sc.next();
	}

} Main static final PrintStream out = System.out; out System.out System System.out static Scanner sc = new Scanner(System.in); sc new Scanner(System.in) static final long Mod = 1000_000_007L; Mod 1000_000_007L public static void main(String[] args) {
		Main main = new Main();
		main.solve();
		System.exit(0);
	} main {
		Main main = new Main();
		main.solve();
		System.exit(0);
	} Main main = new Main(); main new Main() main.solve() main.solve main System.exit(0) System.exit System 0 String[] args args void solve() {
		long A = nextInt();
		long B = nextInt();
		long K = (A+B)/2;

		if(Math.abs(A-K) == Math.abs(B-K)) {
			out.println(K);
		}else{
			out.println("IMPOSSIBLE");
		}
	} solve {
		long A = nextInt();
		long B = nextInt();
		long K = (A+B)/2;

		if(Math.abs(A-K) == Math.abs(B-K)) {
			out.println(K);
		}else{
			out.println("IMPOSSIBLE");
		}
	} long A = nextInt(); A nextInt() nextInt long B = nextInt(); B nextInt() nextInt long K = (A+B)/2; K (A+B)/2 (A+B) A B 2 if(Math.abs(A-K) == Math.abs(B-K)) {
			out.println(K);
		}else{
			out.println("IMPOSSIBLE");
		} Math.abs(A-K) == Math.abs(B-K) Math.abs(A-K) Math.abs Math A-K A K Math.abs(B-K) Math.abs Math B-K B K {
			out.println(K);
		} out.println(K) out.println out K {
			out.println("IMPOSSIBLE");
		} out.println("IMPOSSIBLE") out.println out "IMPOSSIBLE" public int[] nextIntArray(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)a[i] = nextInt();
		return a;
	} nextIntArray {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)a[i] = nextInt();
		return a;
	} int[] a = new int[n]; a new int[n] n for (int i = 0; i < n; i++)a[i] = nextInt(); int i = 0; int i = 0; i 0 i < n i n i++ i++ i a[i] = nextInt(); a[i] = nextInt() a[i] a i nextInt() nextInt return a; a int n n public long[] nextLongArray(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)a[i] = nextLong();
		return a;
	} nextLongArray {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)a[i] = nextLong();
		return a;
	} long[] a = new long[n]; a new long[n] n for (int i = 0; i < n; i++)a[i] = nextLong(); int i = 0; int i = 0; i 0 i < n i n i++ i++ i a[i] = nextLong(); a[i] = nextLong() a[i] a i nextLong() nextLong return a; a int n n public int nextInt() {
		return Integer.parseInt(next());
	} nextInt {
		return Integer.parseInt(next());
	} return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next public long nextLong() {
		return Long.parseLong(next());
	} nextLong {
		return Long.parseLong(next());
	} return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next public String next() {
		return sc.next();
	} next {
		return sc.next();
	} return sc.next(); sc.next() sc.next sc public class Main {
	static final PrintStream out = System.out;
	static Scanner sc = new Scanner(System.in);
	static final long Mod = 1000_000_007L;
	public static void main(String[] args) {
		Main main = new Main();
		main.solve();
		System.exit(0);
	}

	void solve() {
		long A = nextInt();
		long B = nextInt();
		long K = (A+B)/2;

		if(Math.abs(A-K) == Math.abs(B-K)) {
			out.println(K);
		}else{
			out.println("IMPOSSIBLE");
		}
	}

	public int[] nextIntArray(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)a[i] = nextInt();
		return a;
	}
	public long[] nextLongArray(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)a[i] = nextLong();
		return a;
	}
	public int nextInt() {
		return Integer.parseInt(next());
	}
	public long nextLong() {
		return Long.parseLong(next());
	}
	public String next() {
		return sc.next();
	}

} public class Main {
	static final PrintStream out = System.out;
	static Scanner sc = new Scanner(System.in);
	static final long Mod = 1000_000_007L;
	public static void main(String[] args) {
		Main main = new Main();
		main.solve();
		System.exit(0);
	}

	void solve() {
		long A = nextInt();
		long B = nextInt();
		long K = (A+B)/2;

		if(Math.abs(A-K) == Math.abs(B-K)) {
			out.println(K);
		}else{
			out.println("IMPOSSIBLE");
		}
	}

	public int[] nextIntArray(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)a[i] = nextInt();
		return a;
	}
	public long[] nextLongArray(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)a[i] = nextLong();
		return a;
	}
	public int nextInt() {
		return Integer.parseInt(next());
	}
	public long nextLong() {
		return Long.parseLong(next());
	}
	public String next() {
		return sc.next();
	}

} Main
import java.io.PrintStream;
import java.util.Scanner;

public class Main {
	static final PrintStream out = System.out;
	static Scanner sc = new Scanner(System.in);
	static final long Mod = 1000_000_007L;
	public static void main(String[] args) {
		Main main = new Main();
		main.solve();
		System.exit(0);
	}

	void solve() {
		long A = nextInt();
		long B = nextInt();
		long K = (A+B)/2;

		if(Math.abs(A-K) == Math.abs(B-K)) {
			out.println(K);
		}else{
			out.println("IMPOSSIBLE");
		}
	}

	public int[] nextIntArray(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)a[i] = nextInt();
		return a;
	}
	public long[] nextLongArray(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)a[i] = nextLong();
		return a;
	}
	public int nextInt() {
		return Integer.parseInt(next());
	}
	public long nextLong() {
		return Long.parseLong(next());
	}
	public String next() {
		return sc.next();
	}

}