import java.util.*;
import java.io.*;

class Main {

	void solve () {

		int a = in.nextInt(), b = in.nextInt();
		
		if (Math.abs(a-b)%2 == 1) out.println("IMPOSSIBLE");
		else {
			int dif = Math.abs(a-b)/2;
			out.println(Math.max(a,b)-dif);
		}
		
	}

	public static PrintWriter out = new PrintWriter(System.out);
	public static Scanner in = new Scanner(System.in);
	public static void main(String[] args) {
		new Main().solve();
		out.flush();
	}

} import java.util.*; util.* java import java.io.*; io.* java class Main {

	void solve () {

		int a = in.nextInt(), b = in.nextInt();
		
		if (Math.abs(a-b)%2 == 1) out.println("IMPOSSIBLE");
		else {
			int dif = Math.abs(a-b)/2;
			out.println(Math.max(a,b)-dif);
		}
		
	}

	public static PrintWriter out = new PrintWriter(System.out);
	public static Scanner in = new Scanner(System.in);
	public static void main(String[] args) {
		new Main().solve();
		out.flush();
	}

} Main public static PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) public static Scanner in = new Scanner(System.in); in new Scanner(System.in) void solve () {

		int a = in.nextInt(), b = in.nextInt();
		
		if (Math.abs(a-b)%2 == 1) out.println("IMPOSSIBLE");
		else {
			int dif = Math.abs(a-b)/2;
			out.println(Math.max(a,b)-dif);
		}
		
	} solve {

		int a = in.nextInt(), b = in.nextInt();
		
		if (Math.abs(a-b)%2 == 1) out.println("IMPOSSIBLE");
		else {
			int dif = Math.abs(a-b)/2;
			out.println(Math.max(a,b)-dif);
		}
		
	} int a = in.nextInt() a in.nextInt() in.nextInt in b = in.nextInt(); b in.nextInt() in.nextInt in if (Math.abs(a-b)%2 == 1) out.println("IMPOSSIBLE");
		else {
			int dif = Math.abs(a-b)/2;
			out.println(Math.max(a,b)-dif);
		} Math.abs(a-b)%2 == 1 Math.abs(a-b)%2 Math.abs(a-b) Math.abs Math a-b a b 2 1 out.println("IMPOSSIBLE") out.println out "IMPOSSIBLE" {
			int dif = Math.abs(a-b)/2;
			out.println(Math.max(a,b)-dif);
		} int dif = Math.abs(a-b)/2; dif Math.abs(a-b)/2 Math.abs(a-b) Math.abs Math a-b a b 2 out.println(Math.max(a,b)-dif) out.println out Math.max(a,b)-dif Math.max(a,b) Math.max Math a b dif public static void main(String[] args) {
		new Main().solve();
		out.flush();
	} main {
		new Main().solve();
		out.flush();
	} new Main().solve() new Main().solve new Main() out.flush() out.flush out String[] args args
import java.util.*;
import java.io.*;

class Main {

	void solve () {

		int a = in.nextInt(), b = in.nextInt();
		
		if (Math.abs(a-b)%2 == 1) out.println("IMPOSSIBLE");
		else {
			int dif = Math.abs(a-b)/2;
			out.println(Math.max(a,b)-dif);
		}
		
	}

	public static PrintWriter out = new PrintWriter(System.out);
	public static Scanner in = new Scanner(System.in);
	public static void main(String[] args) {
		new Main().solve();
		out.flush();
	}

}