import java.util.Scanner;

public 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) System.out.println(ans);
        else System.out.println("IMPOSSIBLE");
    }
} import java.util.Scanner; Scanner java.util public 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) System.out.println(ans);
        else System.out.println("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) System.out.println(ans);
        else System.out.println("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) System.out.println(ans);
        else System.out.println("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) System.out.println(ans);
        else System.out.println("IMPOSSIBLE"); (a + b) % 2 == 0 (a + b) % 2 (a + b) a b 2 0 System.out.println(ans) System.out.println System.out System System.out ans System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args public 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) System.out.println(ans);
        else System.out.println("IMPOSSIBLE");
    }
} public 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) System.out.println(ans);
        else System.out.println("IMPOSSIBLE");
    }
} Main
import java.util.Scanner;

public 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) System.out.println(ans);
        else System.out.println("IMPOSSIBLE");
    }
}