import java.util.*;

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

class Main {
    public static void main(String args[]) {
        Scanner inp = new Scanner(System.in);
        do_(inp);
        inp.close();
    }
    public static void do_(Scanner inp) {
        int a = inp.nextInt();
        int b = inp.nextInt();
        if ((a + b) % 2 == 0) System.out.println((a + b) / 2);
        else System.out.println("IMPOSSIBLE");
    }
}
