import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.Arrays;

public class Main {
	private static final int MOD = 1_000_000_007;

	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();

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

	public static class Pair {
		private int index;
		private int value;

		public Pair(int index, int value) {
			super();
			this.index = index;
			this.value = value;
		}

		public int getIndex() {
			return index;
		}

		public int getValue() {
			return value;
		}

		@Override
		public String toString() {
			return "[" + index + ":" + value + "]";
		}
	}

	private static boolean isDebug = System.getProperty("sun.desktop") != null;

	private static void debug(Object... o) {
		if (isDebug) {
			System.err.println(Arrays.deepToString(o));
		}
	}

	private static long gcd(long m, long n) {
		long temp;
		while (m % n != 0) {
			temp = n;
			n = m % n;
			m = temp;
		}
		return n;
	}

	private static long lcm(long m, long n) {
		return m / gcd(m, n) * n;
	}

	private static int add(int a, int b) {
		return (a + b) % MOD;
	}

	private static int sub(int a, int b) {
		return (a - b % MOD) % MOD;
	}

	private static int mul(int a, int b) {
		return ((a % MOD) * (b % MOD)) % MOD;
	}

	public static class Scanner {
		private BufferedInputStream inputStream;

		public Scanner(InputStream in) {
			inputStream = new BufferedInputStream(in);
		}

		public char nextChar() throws IOException {
			return (char) skip();
		}

		public int nextInt() throws IOException {
			int num = 0;

			int read = skip();
			do {
				num = num * 10 + (read - 0x30);
			} while ((read = inputStream.read()) > 0x20);

			return num;
		}

		public void fill(int[] a) throws IOException {
			for (int i = 0; i < a.length; i++) {
				a[i] = nextInt();
			}
		}

		public void fill(int[] a, int[] b) throws IOException {
			if (a.length != b.length) {
				throw new IllegalArgumentException();
			}

			for (int i = 0; i < a.length; i++) {
				a[i] = nextInt();
				b[i] = nextInt();
			}
		}

		public long nextLong() throws IOException {
			long num = 0;

			int read = skip();
			do {
				num = num * 10 + (read - 0x30);
			} while ((read = inputStream.read()) > 0x20);

			return num;
		}

		public void fill(long[] a) throws IOException {
			for (int i = 0; i < a.length; i++) {
				a[i] = nextLong();
			}
		}

		public void fill(long[] a, long[] b) throws IOException {
			if (a.length != b.length) {
				throw new IllegalArgumentException();
			}

			for (int i = 0; i < a.length; i++) {
				a[i] = nextLong();
				b[i] = nextLong();
			}
		}

		public long[] nextLong(int n) throws IOException {
			long[] array = new long[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextLong();
			}

			return array;
		}

		public String next() throws IOException {
			StringBuilder builder = new StringBuilder();

			int read = skip();
			do {
				builder.append((char) read);
			} while ((read = inputStream.read()) > 0x20);

			return builder.toString();
		}

		private int skip() throws IOException {
			int read;
			while ((read = inputStream.read()) <= 0x20)
				;

			return read;
		}
	}
}

Measured memory_bytes: 21204