import java.util.*;

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

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