/*
	submission # - User: herp_sy
	https://atcoder.jp/contests/
*/

import java.util.*;
import java.util.Collections; // sorting use
import java.io.*;
import static java.lang.System.*;

public class Main{
	public static void main(String[] args){
		int a,b;
		int k;
		Scanner sc = new Scanner(System.in);
		a = sc.nextInt();
		b = sc.nextInt();
		k = (a + b) / 2;

		if(Math.abs(k - a) == Math.abs(k - b))	out.println(k);
		else out.println("IMPOSSIBLE");
	}
} import java.util.*; util.* java import java.util.Collections; // sorting use Collections java.util import java.io.*; io.* java import static java.lang.System.*; import static java.lang.System.*; import static java.lang.System.*; public class Main{
	public static void main(String[] args){
		int a,b;
		int k;
		Scanner sc = new Scanner(System.in);
		a = sc.nextInt();
		b = sc.nextInt();
		k = (a + b) / 2;

		if(Math.abs(k - a) == Math.abs(k - b))	out.println(k);
		else out.println("IMPOSSIBLE");
	}
} Main public static void main(String[] args){
		int a,b;
		int k;
		Scanner sc = new Scanner(System.in);
		a = sc.nextInt();
		b = sc.nextInt();
		k = (a + b) / 2;

		if(Math.abs(k - a) == Math.abs(k - b))	out.println(k);
		else out.println("IMPOSSIBLE");
	} main {
		int a,b;
		int k;
		Scanner sc = new Scanner(System.in);
		a = sc.nextInt();
		b = sc.nextInt();
		k = (a + b) / 2;

		if(Math.abs(k - a) == Math.abs(k - b))	out.println(k);
		else out.println("IMPOSSIBLE");
	} int a a b; b int k; k Scanner sc = new Scanner(System.in); sc new Scanner(System.in) a = sc.nextInt() a sc.nextInt() sc.nextInt sc b = sc.nextInt() b sc.nextInt() sc.nextInt sc k = (a + b) / 2 k (a + b) / 2 (a + b) a b 2 if(Math.abs(k - a) == Math.abs(k - b))	out.println(k);
		else out.println("IMPOSSIBLE"); Math.abs(k - a) == Math.abs(k - b) Math.abs(k - a) Math.abs Math k - a k a Math.abs(k - b) Math.abs Math k - b k b out.println(k) out.println out k out.println("IMPOSSIBLE") out.println out "IMPOSSIBLE" String[] args args public class Main{
	public static void main(String[] args){
		int a,b;
		int k;
		Scanner sc = new Scanner(System.in);
		a = sc.nextInt();
		b = sc.nextInt();
		k = (a + b) / 2;

		if(Math.abs(k - a) == Math.abs(k - b))	out.println(k);
		else out.println("IMPOSSIBLE");
	}
} public class Main{
	public static void main(String[] args){
		int a,b;
		int k;
		Scanner sc = new Scanner(System.in);
		a = sc.nextInt();
		b = sc.nextInt();
		k = (a + b) / 2;

		if(Math.abs(k - a) == Math.abs(k - b))	out.println(k);
		else out.println("IMPOSSIBLE");
	}
} Main
/*
	submission # - User: herp_sy
	https://atcoder.jp/contests/
*/

import java.util.*;
import java.util.Collections; // sorting use
import java.io.*;
import static java.lang.System.*;

public class Main{
	public static void main(String[] args){
		int a,b;
		int k;
		Scanner sc = new Scanner(System.in);
		a = sc.nextInt();
		b = sc.nextInt();
		k = (a + b) / 2;

		if(Math.abs(k - a) == Math.abs(k - b))	out.println(k);
		else out.println("IMPOSSIBLE");
	}
}
