function Main(input) {

	input = input.split("\n");
	var A = parseInt(input[0]);
	var B = parseInt(input[1]);
	
	var answer;
	
	if(((A+B)/2)%2===0){
	    answer = (A+B)/2;
	}else{
	    answer = 'IMPOSSIBLE';
	}

	
    console.log(answer);

}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));