import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        long a = sc.nextLong();
        long b = sc.nextLong();

        int ans = -1;
        for (int i = 0; i < Math.max(a, b); i++) {
            if (Math.abs(a - i) == Math.abs(b - i)) {
                ans = i;
                break;
            }
        }

        System.out.println(ans < 0 ? "IMPOSSIBLE" : ans);
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        long a = sc.nextLong();
        long b = sc.nextLong();

        int ans = -1;
        for (int i = 0; i < Math.max(a, b); i++) {
            if (Math.abs(a - i) == Math.abs(b - i)) {
                ans = i;
                break;
            }
        }

        System.out.println(ans < 0 ? "IMPOSSIBLE" : ans);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        long a = sc.nextLong();
        long b = sc.nextLong();

        int ans = -1;
        for (int i = 0; i < Math.max(a, b); i++) {
            if (Math.abs(a - i) == Math.abs(b - i)) {
                ans = i;
                break;
            }
        }

        System.out.println(ans < 0 ? "IMPOSSIBLE" : ans);
    } main {
        Scanner sc = new Scanner(System.in);

        long a = sc.nextLong();
        long b = sc.nextLong();

        int ans = -1;
        for (int i = 0; i < Math.max(a, b); i++) {
            if (Math.abs(a - i) == Math.abs(b - i)) {
                ans = i;
                break;
            }
        }

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

        long a = sc.nextLong();
        long b = sc.nextLong();

        int ans = -1;
        for (int i = 0; i < Math.max(a, b); i++) {
            if (Math.abs(a - i) == Math.abs(b - i)) {
                ans = i;
                break;
            }
        }

        System.out.println(ans < 0 ? "IMPOSSIBLE" : ans);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        long a = sc.nextLong();
        long b = sc.nextLong();

        int ans = -1;
        for (int i = 0; i < Math.max(a, b); i++) {
            if (Math.abs(a - i) == Math.abs(b - i)) {
                ans = i;
                break;
            }
        }

        System.out.println(ans < 0 ? "IMPOSSIBLE" : ans);
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        long a = sc.nextLong();
        long b = sc.nextLong();

        int ans = -1;
        for (int i = 0; i < Math.max(a, b); i++) {
            if (Math.abs(a - i) == Math.abs(b - i)) {
                ans = i;
                break;
            }
        }

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