import java.util.*;

class Main
{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		
		int ind = Math.max(a, b);
		
		boolean flag = false;
		int ans = 0;
		
		for(int i = 1; i < ind; ++i) {
		    if(Math.abs(a - i) == Math.abs(b - i)){
		        flag = true;
		        ans = i;
		        break;
		    }
		    else flag = false;
		}
		if(flag) System.out.println(ans);
		else System.out.println("IMPOSSIBLE");
		
	}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz