import java.util.*;
import java.io.*;
public class Main {
	public static void main(String[] args) {
		Solver solver = new Solver();
		solver.solve();
		solver.exit();
	}

	static class FastScanner {
		private final InputStream in = System.in;
		private final byte[] buffer = new byte[1024];
		private int ptr = 0;
		private int buflen = 0;
		private boolean hasNextByte() {
			if (ptr < buflen) {
				return true;
			}else{
				ptr = 0;
				try {
					buflen = in.read(buffer);
				} catch (IOException e) {
					e.printStackTrace();
				}
				if (buflen <= 0) {
					return false;
				}
			}
			return true;
		}
		private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
		private boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
		private void skipUnprintable() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;}
		public boolean hasNext() { skipUnprintable(); return hasNextByte();}
		public String next() {
			if (!hasNext()) throw new NoSuchElementException();
			StringBuilder sb = new StringBuilder();
			int b = readByte();
			while(isPrintableChar(b)) {
				sb.appendCodePoint(b);
				b = readByte();
			}
			return sb.toString();
		}
		public long nextLong() {
			if (!hasNext()) throw new NoSuchElementException();
			long n = 0;
			boolean minus = false;
			int b = readByte();
			if (b == '-') {
				minus = true;
				b = readByte();
			}
			if (b < '0' || '9' < b) {
				throw new NumberFormatException();
			}
			while(true){
				if ('0' <= b && b <= '9') {
					n *= 10;
					n += b - '0';
				}else if(b == -1 || !isPrintableChar(b)){
					return minus ? -n : n;
				}else{
					throw new NumberFormatException();
				}
				b = readByte();
			}
		}
	}

	static class Solver {
		FastScanner sc = new FastScanner();
		public Solver() { }

		String ns() { return sc.next(); }
		String[] ns(int n) {
			String a[] = new String[n];
			for(int i = 0; i < n; i ++) { a[i] = ns(); }
			return a;
		}
		String[][] ns(int n, int m) {
			String a[][] = new String[n][m];
			for(int i = 0; i < n; i ++) { a[i] = ns(m); }
			return a;
		}
		int ni() { return (int)sc.nextLong(); }
		int[] ni(int n) {
			int a[] = new int[n];
			for(int i = 0; i < n; i ++) { a[i] = ni(); }
			return a;
		}
		int[][] ni(int n, int m) {
			int a[][] = new int[n][m];
			for(int i = 0; i < n; i ++) { a[i] = ni(m); }
			return a;
		}
		long nl() { return sc.nextLong(); }
		long[] nl(int n) {
			long a[] = new long[n];
			for(int i = 0; i < n; i ++) { a[i] = nl(); }
			return a;
		}
		long[][] nl(int n, int m) {
			long a[][] = new long[n][m];
			for(int i = 0; i < n; i ++) { a[i] = nl(m); }
			return a;
		}

		PrintWriter out = new PrintWriter(System.out);
		PrintWriter err = new PrintWriter(System.err);
		void prt() { out.print(""); }
		void prt(int a) { out.print(a); }
		void prt(long a) { out.print(a); }
		void prt(double a) { out.print(a); }
		void prt(String a) { out.print(a); }
		void prt(boolean a) { out.print(a); }
		void prt(char a) { out.print(a); }
		void prt(Object a) { out.print(a); }
		void prtln() { out.println(""); }
		void prtln(int a) { out.println(a); }
		void prtln(long a) { out.println(a); }
		void prtln(double a) { out.println(a); }
		void prtln(String a) { out.println(a); }
		void prtln(boolean a) { out.println(a); }
		void prtln(char a) { out.println(a); }
		void prtln(Object a) { out.println(a); }
		void prtln(int... a) { for(int element : a){ prt(element+" "); } prtln(); }
		void prtln(long... a) { for(long element : a){ prt(element+" "); } prtln(); }
		void prtln(double... a) { for(double element : a){ prt(element+" "); } prtln(); }
		void prtln(String... a) { for(String element : a){ prt(element+" "); } prtln(); }
		void prtln(boolean... a) { for(boolean element : a){ prt(element+" "); } prtln(); }
		void prtln(int[][] a) { for(int[] element : a){ prtln(element); } }
		void prtln(long[][] a) { for(long[] element : a){ prtln(element); } }
		void prtln(double[][] a) { for(double[] element : a){ prtln(element); } }
		void prtln(String[][] a) { for(String[] element : a){ prtln(element); } }
		void prtln(boolean[][] a) { for(boolean[] element : a){ prtln(element); } }
		void errprt() { err.print(""); }
		void errprt(int a) { err.print(a); }
		void errprt(long a) { err.print(a); }
		void errprt(double a) { err.print(a); }
		void errprt(String a) { err.print(a); }
		void errprt(boolean a) { err.println(a); }
		void errprt(char a) { err.println(a); }
		void errprt(Object a) { err.println(a); }
		void errprtln() { err.println(""); }
		void errprtln(int a) { err.println(a); }
		void errprtln(long a) { err.println(a); }
		void errprtln(double a) { err.println(a); }
		void errprtln(String a) { err.println(a); }
		void errprtln(boolean a) { err.println(a); }
		void errprtln(char a) { err.println(a); }
		void errprtln(Object a) { err.println(a); }
		void errprtln(int... a) { for(int element : a){ errprt(element+" "); } errprtln(); }
		void errprtln(long... a) { for(long element : a){ errprt(element+" "); } errprtln(); }
		void errprtln(double... a) { for(double element : a){ errprt(element+" "); } errprtln(); }
		void errprtln(String... a) { for(String element : a){ errprt(element+" "); } errprtln(); }
		void errprtln(boolean... a) { for(boolean element : a){ errprt(element+" "); } errprtln(); }
		void errprtln(boolean[][] a) { for(boolean[] element : a){ errprtln(element); } }
		void reply(boolean b) { prtln(b ? "Yes" : "No"); }
		void REPLY(boolean b) { prtln(b ? "YES" : "NO"); }

		void exit() { out.flush(); err.flush(); System.exit(0); }

		int min(int a, int b) { return Math.min(a, b); }
		long min(long a, long b) { return Math.min(a, b); }
		double min(double a, double b) { return Math.min(a, b); }
		int min(int... x) {
			int min = x[0];
			for(int val : x) { min = min(min, val); }
			return min;
		}
		long min(long... x) {
			long min = x[0];
			for(long val : x) { min = min(min, val); }
			return min;
		}
		double min(double... x) {
			double min = x[0];
			for(double val : x) { min = min(min, val); }
			return min;
		}
		int max(int a, int b) { return Math.max(a, b); }
		long max(long a, long b) { return Math.max(a, b); }
		double max(double a, double b) { return Math.max(a, b); }
		int max(int... x) {
			int max = x[0];
			for(int val : x) { max = max(max, val); }
			return max;
		}
		long max(long... x) {
			long max = x[0];
			for(long val : x) { max = max(max, val); }
			return max;
		}
		double max(double... x) {
			double max = x[0];
			for(double val : x) { max = max(max, val); }
			return max;
		}
		long sum(int... a) {
			long sum = 0;
			for(int element : a) { sum += element; }
			return sum;
		}
		long sum(long... a) {
			long sum = 0;
			for(long element : a) { sum += element; }
			return sum;
		}
		double sum(double... a) {
			double sum = 0;
			for(double element : a) { sum += element; }
			return sum;
		}

		long abs(double x) { return (long)Math.abs(x); }
		long round(double x) { return Math.round(x); }
		long floor(double x) { return (long)Math.floor(x); }
		long ceil(double x) { return (long)Math.ceil(x); }
		double sqrt(double x) { return Math.sqrt(x); }
		double pow(double x, double y) { return Math.pow(x, y); }
		long pow(long x, long y) { return (long)Math.pow(x, y); }
		int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
		long gcd(long a, long b) { return b == 0 ? a : gcd(b, a % b); }

		public void solve() {
			int a = ni();
			int b = ni();
			prtln((a + b) % 2 == 0 ? (a + b) / 2 : "IMPOSSIBLE");
		}
	}
}

Measured memory_bytes: 20820