import java.io.*;

public class Main {

	public static void main(String[] args) throws IOException{
		// TODO Auto-generated method stub
		InputStreamReader ir=new InputStreamReader(System.in);
		BufferedReader br=new BufferedReader(ir);
		String [] num=br.readLine().split(" ");
		int A=Integer.parseInt(num[0]);
		int B=Integer.parseInt(num[1]);
		int sum=A+B;
		if(A == B)
		{
			System.out.println(0);
		}
		else if(sum % 2 !=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			int k=sum/2;
			System.out.println(k);
		}
		
	}

} import java.io.*; io.* java public class Main {

	public static void main(String[] args) throws IOException{
		// TODO Auto-generated method stub
		InputStreamReader ir=new InputStreamReader(System.in);
		BufferedReader br=new BufferedReader(ir);
		String [] num=br.readLine().split(" ");
		int A=Integer.parseInt(num[0]);
		int B=Integer.parseInt(num[1]);
		int sum=A+B;
		if(A == B)
		{
			System.out.println(0);
		}
		else if(sum % 2 !=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			int k=sum/2;
			System.out.println(k);
		}
		
	}

} Main public static void main(String[] args) throws IOException{
		// TODO Auto-generated method stub
		InputStreamReader ir=new InputStreamReader(System.in);
		BufferedReader br=new BufferedReader(ir);
		String [] num=br.readLine().split(" ");
		int A=Integer.parseInt(num[0]);
		int B=Integer.parseInt(num[1]);
		int sum=A+B;
		if(A == B)
		{
			System.out.println(0);
		}
		else if(sum % 2 !=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			int k=sum/2;
			System.out.println(k);
		}
		
	} main {
		// TODO Auto-generated method stub
		InputStreamReader ir=new InputStreamReader(System.in);
		BufferedReader br=new BufferedReader(ir);
		String [] num=br.readLine().split(" ");
		int A=Integer.parseInt(num[0]);
		int B=Integer.parseInt(num[1]);
		int sum=A+B;
		if(A == B)
		{
			System.out.println(0);
		}
		else if(sum % 2 !=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			int k=sum/2;
			System.out.println(k);
		}
		
	} InputStreamReader ir=new InputStreamReader(System.in); ir new InputStreamReader(System.in) BufferedReader br=new BufferedReader(ir); br new BufferedReader(ir) String [] num=br.readLine().split(" "); num br.readLine().split(" ") br.readLine().split br.readLine() br.readLine br " " int A=Integer.parseInt(num[0]); A Integer.parseInt(num[0]) Integer.parseInt Integer num[0] num 0 int B=Integer.parseInt(num[1]); B Integer.parseInt(num[1]) Integer.parseInt Integer num[1] num 1 int sum=A+B; sum A+B A B if(A == B)
		{
			System.out.println(0);
		}
		else if(sum % 2 !=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			int k=sum/2;
			System.out.println(k);
		} A == B A B {
			System.out.println(0);
		} System.out.println(0) System.out.println System.out System System.out 0 if(sum % 2 !=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			int k=sum/2;
			System.out.println(k);
		} sum % 2 !=0 sum % 2 sum 2 0 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			int k=sum/2;
			System.out.println(k);
		} int k=sum/2; k sum/2 sum 2 System.out.println(k) System.out.println System.out System System.out k String[] args args public class Main {

	public static void main(String[] args) throws IOException{
		// TODO Auto-generated method stub
		InputStreamReader ir=new InputStreamReader(System.in);
		BufferedReader br=new BufferedReader(ir);
		String [] num=br.readLine().split(" ");
		int A=Integer.parseInt(num[0]);
		int B=Integer.parseInt(num[1]);
		int sum=A+B;
		if(A == B)
		{
			System.out.println(0);
		}
		else if(sum % 2 !=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			int k=sum/2;
			System.out.println(k);
		}
		
	}

} public class Main {

	public static void main(String[] args) throws IOException{
		// TODO Auto-generated method stub
		InputStreamReader ir=new InputStreamReader(System.in);
		BufferedReader br=new BufferedReader(ir);
		String [] num=br.readLine().split(" ");
		int A=Integer.parseInt(num[0]);
		int B=Integer.parseInt(num[1]);
		int sum=A+B;
		if(A == B)
		{
			System.out.println(0);
		}
		else if(sum % 2 !=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			int k=sum/2;
			System.out.println(k);
		}
		
	}

} Main
import java.io.*;

public class Main {

	public static void main(String[] args) throws IOException{
		// TODO Auto-generated method stub
		InputStreamReader ir=new InputStreamReader(System.in);
		BufferedReader br=new BufferedReader(ir);
		String [] num=br.readLine().split(" ");
		int A=Integer.parseInt(num[0]);
		int B=Integer.parseInt(num[1]);
		int sum=A+B;
		if(A == B)
		{
			System.out.println(0);
		}
		else if(sum % 2 !=0)
		{
			System.out.println("IMPOSSIBLE");
		}
		else
		{
			int k=sum/2;
			System.out.println(k);
		}
		
	}

}
