import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int a = input.nextInt();
        int b = input.nextInt();
        int ans = -1;
        for(int i = 0; i <= Math.max(a, b); ++i){
            int x = Math.abs(a - i);
            int y = Math.abs(b - i);
            if(x == y) {ans = i; break;}
        }
        if(ans == -1) System.out.println("IMPOSSIBLE");
        else System.out.println(ans);
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int a = input.nextInt();
        int b = input.nextInt();
        int ans = -1;
        for(int i = 0; i <= Math.max(a, b); ++i){
            int x = Math.abs(a - i);
            int y = Math.abs(b - i);
            if(x == y) {ans = i; break;}
        }
        if(ans == -1) System.out.println("IMPOSSIBLE");
        else System.out.println(ans);
    }
} Main public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int a = input.nextInt();
        int b = input.nextInt();
        int ans = -1;
        for(int i = 0; i <= Math.max(a, b); ++i){
            int x = Math.abs(a - i);
            int y = Math.abs(b - i);
            if(x == y) {ans = i; break;}
        }
        if(ans == -1) System.out.println("IMPOSSIBLE");
        else System.out.println(ans);
    } main {
        Scanner input = new Scanner(System.in);
        int a = input.nextInt();
        int b = input.nextInt();
        int ans = -1;
        for(int i = 0; i <= Math.max(a, b); ++i){
            int x = Math.abs(a - i);
            int y = Math.abs(b - i);
            if(x == y) {ans = i; break;}
        }
        if(ans == -1) System.out.println("IMPOSSIBLE");
        else System.out.println(ans);
    } Scanner input = new Scanner(System.in); input new Scanner(System.in) int a = input.nextInt(); a input.nextInt() input.nextInt input int b = input.nextInt(); b input.nextInt() input.nextInt input int ans = -1; ans -1 1 for(int i = 0; i <= Math.max(a, b); ++i){
            int x = Math.abs(a - i);
            int y = Math.abs(b - i);
            if(x == y) {ans = i; break;}
        } int i = 0; int i = 0; i 0 i <= Math.max(a, b) i Math.max(a, b) Math.max Math a b ++i ++i i {
            int x = Math.abs(a - i);
            int y = Math.abs(b - i);
            if(x == y) {ans = i; break;}
        } {
            int x = Math.abs(a - i);
            int y = Math.abs(b - i);
            if(x == y) {ans = i; break;}
        } int x = Math.abs(a - i); x Math.abs(a - i) Math.abs Math a - i a i int y = Math.abs(b - i); y Math.abs(b - i) Math.abs Math b - i b i if(x == y) {ans = i; break;} x == y x y {ans = i; break;} ans = i ans i break; if(ans == -1) System.out.println("IMPOSSIBLE");
        else System.out.println(ans); ans == -1 ans -1 1 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int a = input.nextInt();
        int b = input.nextInt();
        int ans = -1;
        for(int i = 0; i <= Math.max(a, b); ++i){
            int x = Math.abs(a - i);
            int y = Math.abs(b - i);
            if(x == y) {ans = i; break;}
        }
        if(ans == -1) System.out.println("IMPOSSIBLE");
        else System.out.println(ans);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int a = input.nextInt();
        int b = input.nextInt();
        int ans = -1;
        for(int i = 0; i <= Math.max(a, b); ++i){
            int x = Math.abs(a - i);
            int y = Math.abs(b - i);
            if(x == y) {ans = i; break;}
        }
        if(ans == -1) System.out.println("IMPOSSIBLE");
        else System.out.println(ans);
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int a = input.nextInt();
        int b = input.nextInt();
        int ans = -1;
        for(int i = 0; i <= Math.max(a, b); ++i){
            int x = Math.abs(a - i);
            int y = Math.abs(b - i);
            if(x == y) {ans = i; break;}
        }
        if(ans == -1) System.out.println("IMPOSSIBLE");
        else System.out.println(ans);
    }
}
