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

public class Main{
	public static void main (String[] args){
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		System.out.println((b-a)%2 == 0 ? (b+a)/2 : "IMPOSSIBLE");
	}
} import java.util.*; util.* java import java.lang.*; lang.* java import java.io.*; io.* java public class Main{
	public static void main (String[] args){
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		System.out.println((b-a)%2 == 0 ? (b+a)/2 : "IMPOSSIBLE");
	}
} Main public static void main (String[] args){
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		System.out.println((b-a)%2 == 0 ? (b+a)/2 : "IMPOSSIBLE");
	} main {
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		System.out.println((b-a)%2 == 0 ? (b+a)/2 : "IMPOSSIBLE");
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = Integer.parseInt(sc.next()); a Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int b = Integer.parseInt(sc.next()); b Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc System.out.println((b-a)%2 == 0 ? (b+a)/2 : "IMPOSSIBLE") System.out.println System.out System System.out (b-a)%2 == 0 ? (b+a)/2 : "IMPOSSIBLE" (b-a)%2 == 0 (b-a)%2 (b-a) b a 2 0 (b+a)/2 (b+a) b a 2 "IMPOSSIBLE" String[] args args public class Main{
	public static void main (String[] args){
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		System.out.println((b-a)%2 == 0 ? (b+a)/2 : "IMPOSSIBLE");
	}
} public class Main{
	public static void main (String[] args){
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		System.out.println((b-a)%2 == 0 ? (b+a)/2 : "IMPOSSIBLE");
	}
} Main
import java.util.*;
import java.lang.*;
import java.io.*;

public class Main{
	public static void main (String[] args){
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		System.out.println((b-a)%2 == 0 ? (b+a)/2 : "IMPOSSIBLE");
	}
}
