import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String line;
		String in[] = new String[2];
		int a= 0, b = 0;
		try {
			line = br.readLine();
			in = line.split(" ");
			a = Integer.parseInt(in[0]);
			b = Integer.parseInt(in[1]);
		} catch (IOException e) {
			e.getStackTrace();
		}
		
		double ans = (a + b) /2.0;
		double x = Math.ceil(ans);
		if(ans == x) {
			System.out.println((int)x);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io public class Main {
	public static void main(String[] args) {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String line;
		String in[] = new String[2];
		int a= 0, b = 0;
		try {
			line = br.readLine();
			in = line.split(" ");
			a = Integer.parseInt(in[0]);
			b = Integer.parseInt(in[1]);
		} catch (IOException e) {
			e.getStackTrace();
		}
		
		double ans = (a + b) /2.0;
		double x = Math.ceil(ans);
		if(ans == x) {
			System.out.println((int)x);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main public static void main(String[] args) {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String line;
		String in[] = new String[2];
		int a= 0, b = 0;
		try {
			line = br.readLine();
			in = line.split(" ");
			a = Integer.parseInt(in[0]);
			b = Integer.parseInt(in[1]);
		} catch (IOException e) {
			e.getStackTrace();
		}
		
		double ans = (a + b) /2.0;
		double x = Math.ceil(ans);
		if(ans == x) {
			System.out.println((int)x);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	} main {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String line;
		String in[] = new String[2];
		int a= 0, b = 0;
		try {
			line = br.readLine();
			in = line.split(" ");
			a = Integer.parseInt(in[0]);
			b = Integer.parseInt(in[1]);
		} catch (IOException e) {
			e.getStackTrace();
		}
		
		double ans = (a + b) /2.0;
		double x = Math.ceil(ans);
		if(ans == x) {
			System.out.println((int)x);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	} BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) String line; line String in[] = new String[2]; in new String[2] 2 int a= 0 a 0 b = 0; b 0 try {
			line = br.readLine();
			in = line.split(" ");
			a = Integer.parseInt(in[0]);
			b = Integer.parseInt(in[1]);
		} catch (IOException e) {
			e.getStackTrace();
		} catch (IOException e) {
			e.getStackTrace();
		} IOException e {
			e.getStackTrace();
		} e.getStackTrace() e.getStackTrace e {
			line = br.readLine();
			in = line.split(" ");
			a = Integer.parseInt(in[0]);
			b = Integer.parseInt(in[1]);
		} line = br.readLine() line br.readLine() br.readLine br in = line.split(" ") in line.split(" ") line.split line " " a = Integer.parseInt(in[0]) a Integer.parseInt(in[0]) Integer.parseInt Integer in[0] in 0 b = Integer.parseInt(in[1]) b Integer.parseInt(in[1]) Integer.parseInt Integer in[1] in 1 double ans = (a + b) /2.0; ans (a + b) /2.0 (a + b) a b 2.0 double x = Math.ceil(ans); x Math.ceil(ans) Math.ceil Math ans if(ans == x) {
			System.out.println((int)x);
		} else {
			System.out.println("IMPOSSIBLE");
		} ans == x ans x {
			System.out.println((int)x);
		} System.out.println((int)x) System.out.println System.out System System.out (int)x {
			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) {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String line;
		String in[] = new String[2];
		int a= 0, b = 0;
		try {
			line = br.readLine();
			in = line.split(" ");
			a = Integer.parseInt(in[0]);
			b = Integer.parseInt(in[1]);
		} catch (IOException e) {
			e.getStackTrace();
		}
		
		double ans = (a + b) /2.0;
		double x = Math.ceil(ans);
		if(ans == x) {
			System.out.println((int)x);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} public class Main {
	public static void main(String[] args) {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String line;
		String in[] = new String[2];
		int a= 0, b = 0;
		try {
			line = br.readLine();
			in = line.split(" ");
			a = Integer.parseInt(in[0]);
			b = Integer.parseInt(in[1]);
		} catch (IOException e) {
			e.getStackTrace();
		}
		
		double ans = (a + b) /2.0;
		double x = Math.ceil(ans);
		if(ans == x) {
			System.out.println((int)x);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String line;
		String in[] = new String[2];
		int a= 0, b = 0;
		try {
			line = br.readLine();
			in = line.split(" ");
			a = Integer.parseInt(in[0]);
			b = Integer.parseInt(in[1]);
		} catch (IOException e) {
			e.getStackTrace();
		}
		
		double ans = (a + b) /2.0;
		double x = Math.ceil(ans);
		if(ans == x) {
			System.out.println((int)x);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
}