import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Akash
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        AHarmony solver = new AHarmony();
        solver.solve(1, in, out);
        out.close();
    }

    static class AHarmony {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            long a = in.nextLong();
            long b = in.nextLong();
            if ((a + b) % 2 == 0) {
                out.println((a + b) / 2);
            } else {
                out.println("IMPOSSIBLE");
            }
        }

    }
}
 import java.io.OutputStream; OutputStream java.io import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        AHarmony solver = new AHarmony();
        solver.solve(1, in, out);
        out.close();
    }

    static class AHarmony {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            long a = in.nextLong();
            long b = in.nextLong();
            if ((a + b) % 2 == 0) {
                out.println((a + b) / 2);
            } else {
                out.println("IMPOSSIBLE");
            }
        }

    }
} Main public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        AHarmony solver = new AHarmony();
        solver.solve(1, in, out);
        out.close();
    } main {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        AHarmony solver = new AHarmony();
        solver.solve(1, in, out);
        out.close();
    } InputStream inputStream = System.in; inputStream System.in System System.in OutputStream outputStream = System.out; outputStream System.out System System.out Scanner in = new Scanner(inputStream); in new Scanner(inputStream) PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) AHarmony solver = new AHarmony(); solver new AHarmony() solver.solve(1, in, out) solver.solve solver 1 in out out.close() out.close out String[] args args static class AHarmony {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            long a = in.nextLong();
            long b = in.nextLong();
            if ((a + b) % 2 == 0) {
                out.println((a + b) / 2);
            } else {
                out.println("IMPOSSIBLE");
            }
        }

    } AHarmony public void solve(int testNumber, Scanner in, PrintWriter out) {
            long a = in.nextLong();
            long b = in.nextLong();
            if ((a + b) % 2 == 0) {
                out.println((a + b) / 2);
            } else {
                out.println("IMPOSSIBLE");
            }
        } solve {
            long a = in.nextLong();
            long b = in.nextLong();
            if ((a + b) % 2 == 0) {
                out.println((a + b) / 2);
            } else {
                out.println("IMPOSSIBLE");
            }
        } long a = in.nextLong(); a in.nextLong() in.nextLong in long b = in.nextLong(); b in.nextLong() in.nextLong in if ((a + b) % 2 == 0) {
                out.println((a + b) / 2);
            } else {
                out.println("IMPOSSIBLE");
            } (a + b) % 2 == 0 (a + b) % 2 (a + b) a b 2 0 {
                out.println((a + b) / 2);
            } out.println((a + b) / 2) out.println out (a + b) / 2 (a + b) a b 2 {
                out.println("IMPOSSIBLE");
            } out.println("IMPOSSIBLE") out.println out "IMPOSSIBLE" int testNumber testNumber Scanner in in PrintWriter out out public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        AHarmony solver = new AHarmony();
        solver.solve(1, in, out);
        out.close();
    }

    static class AHarmony {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            long a = in.nextLong();
            long b = in.nextLong();
            if ((a + b) % 2 == 0) {
                out.println((a + b) / 2);
            } else {
                out.println("IMPOSSIBLE");
            }
        }

    }
} public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        AHarmony solver = new AHarmony();
        solver.solve(1, in, out);
        out.close();
    }

    static class AHarmony {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            long a = in.nextLong();
            long b = in.nextLong();
            if ((a + b) % 2 == 0) {
                out.println((a + b) / 2);
            } else {
                out.println("IMPOSSIBLE");
            }
        }

    }
} Main
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Akash
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        AHarmony solver = new AHarmony();
        solver.solve(1, in, out);
        out.close();
    }

    static class AHarmony {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            long a = in.nextLong();
            long b = in.nextLong();
            if ((a + b) % 2 == 0) {
                out.println((a + b) / 2);
            } else {
                out.println("IMPOSSIBLE");
            }
        }

    }
}

