import java.util.*;

class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int ans = (a+b)/2;
      if((a+b)%2 != 0)
        ans = 0;
        

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

        System.out.println(ans != 0?ans:"IMPOSSIBLE");
    }
} Main public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int ans = (a+b)/2;
      if((a+b)%2 != 0)
        ans = 0;
        

        System.out.println(ans != 0?ans:"IMPOSSIBLE");
    } main {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int ans = (a+b)/2;
      if((a+b)%2 != 0)
        ans = 0;
        

        System.out.println(ans != 0?ans:"IMPOSSIBLE");
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc int b = sc.nextInt(); b sc.nextInt() sc.nextInt sc int ans = (a+b)/2; ans (a+b)/2 (a+b) a b 2 if((a+b)%2 != 0)
        ans = 0; (a+b)%2 != 0 (a+b)%2 (a+b) a b 2 0 ans = 0 ans 0 System.out.println(ans != 0?ans:"IMPOSSIBLE") System.out.println System.out System System.out ans != 0?ans:"IMPOSSIBLE" ans != 0 ans 0 ans "IMPOSSIBLE" String[] args args
import java.util.*;

class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int ans = (a+b)/2;
      if((a+b)%2 != 0)
        ans = 0;
        

        System.out.println(ans != 0?ans:"IMPOSSIBLE");
    }
}
