import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;

public class Main {

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

		int a = 0, b = 0;

		try {
			String[] s = scanner.nextLine().split(" ");
			a = Integer.parseInt(s[0]);
			b = Integer.parseInt(s[1]);
		} catch (NumberFormatException e) {
		} catch (NullPointerException e) {
		}

		scanner.close();
		
		if (((a - b) % 2) != 0) {
			System.out.println("IMPOSSIBLE");
		} else {
		 	int v = Math.abs (a - b) / 2;
		 	
		 	int ret;
		 	if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	}
		 	System.out.println(ret);
		}
	}
} import java.util.Arrays; Arrays java.util import java.util.List; List java.util import java.util.Scanner; Scanner java.util import java.util.stream.Collectors; Collectors java.util.stream public class Main {

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

		int a = 0, b = 0;

		try {
			String[] s = scanner.nextLine().split(" ");
			a = Integer.parseInt(s[0]);
			b = Integer.parseInt(s[1]);
		} catch (NumberFormatException e) {
		} catch (NullPointerException e) {
		}

		scanner.close();
		
		if (((a - b) % 2) != 0) {
			System.out.println("IMPOSSIBLE");
		} else {
		 	int v = Math.abs (a - b) / 2;
		 	
		 	int ret;
		 	if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	}
		 	System.out.println(ret);
		}
	}
} Main public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);

		int a = 0, b = 0;

		try {
			String[] s = scanner.nextLine().split(" ");
			a = Integer.parseInt(s[0]);
			b = Integer.parseInt(s[1]);
		} catch (NumberFormatException e) {
		} catch (NullPointerException e) {
		}

		scanner.close();
		
		if (((a - b) % 2) != 0) {
			System.out.println("IMPOSSIBLE");
		} else {
		 	int v = Math.abs (a - b) / 2;
		 	
		 	int ret;
		 	if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	}
		 	System.out.println(ret);
		}
	} main {
		Scanner scanner = new Scanner(System.in);

		int a = 0, b = 0;

		try {
			String[] s = scanner.nextLine().split(" ");
			a = Integer.parseInt(s[0]);
			b = Integer.parseInt(s[1]);
		} catch (NumberFormatException e) {
		} catch (NullPointerException e) {
		}

		scanner.close();
		
		if (((a - b) % 2) != 0) {
			System.out.println("IMPOSSIBLE");
		} else {
		 	int v = Math.abs (a - b) / 2;
		 	
		 	int ret;
		 	if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	}
		 	System.out.println(ret);
		}
	} Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int a = 0 a 0 b = 0; b 0 try {
			String[] s = scanner.nextLine().split(" ");
			a = Integer.parseInt(s[0]);
			b = Integer.parseInt(s[1]);
		} catch (NumberFormatException e) {
		} catch (NullPointerException e) {
		} catch (NumberFormatException e) {
		} NumberFormatException e {
		} catch (NullPointerException e) {
		} NullPointerException e {
		} {
			String[] s = scanner.nextLine().split(" ");
			a = Integer.parseInt(s[0]);
			b = Integer.parseInt(s[1]);
		} String[] s = scanner.nextLine().split(" "); s scanner.nextLine().split(" ") scanner.nextLine().split scanner.nextLine() scanner.nextLine scanner " " a = Integer.parseInt(s[0]) a Integer.parseInt(s[0]) Integer.parseInt Integer s[0] s 0 b = Integer.parseInt(s[1]) b Integer.parseInt(s[1]) Integer.parseInt Integer s[1] s 1 scanner.close() scanner.close scanner if (((a - b) % 2) != 0) {
			System.out.println("IMPOSSIBLE");
		} else {
		 	int v = Math.abs (a - b) / 2;
		 	
		 	int ret;
		 	if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	}
		 	System.out.println(ret);
		} ((a - b) % 2) != 0 ((a - b) % 2) (a - b) a b 2 0 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
		 	int v = Math.abs (a - b) / 2;
		 	
		 	int ret;
		 	if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	}
		 	System.out.println(ret);
		} int v = Math.abs (a - b) / 2; v Math.abs (a - b) / 2 Math.abs (a - b) Math.abs Math a - b a b 2 int ret; ret if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	} a > b a b {
		 		ret = b + v;
		 	} ret = b + v ret b + v b v {
		 		ret = a + v;
		 	} ret = a + v ret a + v a v System.out.println(ret) System.out.println System.out System System.out ret String[] args args public class Main {

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

		int a = 0, b = 0;

		try {
			String[] s = scanner.nextLine().split(" ");
			a = Integer.parseInt(s[0]);
			b = Integer.parseInt(s[1]);
		} catch (NumberFormatException e) {
		} catch (NullPointerException e) {
		}

		scanner.close();
		
		if (((a - b) % 2) != 0) {
			System.out.println("IMPOSSIBLE");
		} else {
		 	int v = Math.abs (a - b) / 2;
		 	
		 	int ret;
		 	if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	}
		 	System.out.println(ret);
		}
	}
} public class Main {

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

		int a = 0, b = 0;

		try {
			String[] s = scanner.nextLine().split(" ");
			a = Integer.parseInt(s[0]);
			b = Integer.parseInt(s[1]);
		} catch (NumberFormatException e) {
		} catch (NullPointerException e) {
		}

		scanner.close();
		
		if (((a - b) % 2) != 0) {
			System.out.println("IMPOSSIBLE");
		} else {
		 	int v = Math.abs (a - b) / 2;
		 	
		 	int ret;
		 	if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	}
		 	System.out.println(ret);
		}
	}
} Main
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;

public class Main {

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

		int a = 0, b = 0;

		try {
			String[] s = scanner.nextLine().split(" ");
			a = Integer.parseInt(s[0]);
			b = Integer.parseInt(s[1]);
		} catch (NumberFormatException e) {
		} catch (NullPointerException e) {
		}

		scanner.close();
		
		if (((a - b) % 2) != 0) {
			System.out.println("IMPOSSIBLE");
		} else {
		 	int v = Math.abs (a - b) / 2;
		 	
		 	int ret;
		 	if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	}
		 	System.out.println(ret);
		}
	}
}
